Road bike brakes

If you are skidding then that means the brakes are powerful enough to lock the wheel up and your tyres have lost traction, therefore I would suggest the amount of power in the brakes is not the issue.

You have to learn how to modulate your braking, some better pads might help in some situations but won't stop you locking up/skidding when you apply a lot of pressure, that's down to the tyre/surface interface.

Taking a bit of a different line, if you are skidding then wider tyres at lower pressure might help.

Mirroring previous comments, I went from (cheap) Tetkro's to 105's and it proved a very significant upgrade. I plan to go up to SwissStop / Koolstop pads once the stock ones wear out for a further boost. Ultegra/Dura-Ace seems to me to be more off a weight than a performance upgrade, though the newer 9100/8000 models are probably do offer up a bit of a generational improvement. Though at that point I'd rather set the money towards a disc bike. But generally, better braking is a definitely a good "pain saved for money spent" proposition, at least for my money.

Is your bike new, or secondhand?

Repeating the above and adding in a couple of new points about cables:

- change brake pads (I've always found Koolstop Salmon more than enough.)

- clean rims and pads (not applicable if new).

- check the pads are aligned properly on the rim, and also equidistant from the rim on both sides. Much woolliness when braking is caused by one side making contact before the other. Check both calipers are springing back freely.

- adjust the brake cable tension so the bite point is equal for front and back - about a third in of the total lever movement. While you're there, you could do the following...

- check the brake cables are running freely, and that the ends of the outers are finished properly and not catching the inner wire. (My new bike this year had this problem.) If your bike is secondhand, it would probably benefit from new cables.

Too long cables - Some new and secondhand bikes come with cables that are far, far too long.

I'm guessing this is because it gives leeway for fitting longer stems without doing a total cable overhaul. However, sloppy cables negatively affect braking performance. If the stem has been shortened, the cables will almost certainly be too long. I'm pretty certain most bike shops won't have addressed this issue after doing a fitting. Post a pic here if you're not sure if they're the right length. 

Similarly, outer cables can be too short, which also impacts braking and is dangerous.

If it's still shit after doing the above, I'd look at changing the calipers. You say they're Tiagra, but are you totally sure? Is 'Tiagra' visibly written on them? As mentioned above, it's common to have a Tiagra bike with cheaper calipers.

I've got Koolstop salmon ones on my commuting bike and they are very good - the Swiss Stop green ones were out of stock everywhere for Shimano at the time.

I'm going to put the same ones on my other bike when I get time.

I like the discobrakes pads. They're progressive, soft (so don't eat the rims too fast) yet quite long-lasting. Oh, and they're cheap. Even on long drop brakes I can stop as quickly as you'd ever need to.

I upgraded the groupset on my bike from ST2300 (with tektro calipers) to  Tiagra 4700.  I had previously upgraded the brake pads from the Tektro originals to Swiss Stop Green but I decided to use the Shimano Tiagra brake blocks instead when swapped the calipers outs. After a week of bad braking and scored rims I was rooting around in the parts bins for those Swiss Stops.  

The Shimano Tiagra pads had a very wooden on or  off feel and were really bad at trapping grit resulting in some nasty scores on rim, a problem I've not not had with the swiss stops.

Change the pads first, then try upgrading the brakes.   

Decent brake calipers were a revalation to me, I went from non series shimano's to 105's to Ultegra, the difference between the Ultegra's and the 105's was huge and made all the difference. 

Swiss stop pads will cost £20 from anywhere, a new set of Ultegra calipers including pads will be about £80 and is worth spending on IMO.

I have not!

I've just replaced my Dura Ace 7700 group-set with 6800, largely to improve the braking.  The tech on the 7700 and Tiagra is similar, so I'd just change it. Swiss pads didn't make a lot of difference: your problem is mechanical leverage and this can't be improved much by adding more frictional capacity (i.e. better pads).    From what I read the 105 brakes are the same tech as 6800, so buy them instead if you don't mind black/dark grey.  They really do improve your stopping....

I'd echo the above - before you spend more on new calipers, try some Swissstop Greens first, they will be lightyears apart from the factory fitted pads.

It's also worth just checking the fit - that the pads are properly lined up with the braking track, and that the cable is properly tensioned.

I'd say that brake calipers are likely to be the least worthy upgrade in a groupset in terms of performance gain for money spent.

Have a look at Swiss Stop Green pads. They're expensive and don't last very long in the grime of winter but they are absolutely brilliant pads. Great stopping and tons of modulation.

I replaced a friend's inferior brake calipers with 5800 ones.  They were much better afterwards, and only £1 per caliper dearer online than replacement 4700 Tiagra ones. 

She had Tektro calipers from new - the rest of the bike had 4700 equipment - but reviews of the 4700 kit suggested that the calipers were the weakest link in the groupset.  Worth the upgrade to 5800. 

The pads that came with the Tiagra brakes on my old bike were really not very good - descending in the wet felt scary as anything.

I replaced them with some Koolstop salmon ones which were a marked improvement.  I also ended up upgrading to 105 when the Tiagra ones gave up the ghost after a few winters, which again improved things a little.