Best GPS map handlebars device

this looks like quite a good option for a mount - obviously not waterproof, but should hold most phones...

http://photojojo.com/store/awesomeness/handleband/

I recommend an Android smartphone with OsmAnd installed and the offline vector maps downloaded. I fashioned a rudimentary case (not sure how waterproof it is since the screen has to show) out of a mountain bike inner tube and strapped it to my stem with a velcro strap. With a 10000mAh Anker battery pack in a pocket or bag, the battery life can be extended for days.

iPad?...assume you meant iPhone? Suppose you could fashion a way of putting an iPad on top of a Bar Bag, possibly one with a map pocket on it but then you may not be able to work the screen. iPhone works well with the Viewranger App, although if you keep looking at the screen it'll burn up the battery in no time. Some good forum topics on maximising battery life, as well as use of Garmins. As far as I know the Wahoo computer just has speed, dist etc, but a display that has turn by turn navigation (and possibly maps?) can't be that far away (smartphone doing the tracking/nav).

Mobile phone in a Topeak phone case is what you describe, maybe with a booster battery pack. Garmin 705 (second hand) or 800/810/1000 with maps would also work, but probably not to your satisfaction (now that you've been spoiled by such great maps on the phones, those devices seem primitive by comparison).

As you have described Mobile phone with Google maps in cycle mode works very well but lacks battery life.

I use a Garmin 800 with OSM loaded onto sd.

But with Anker battery pack attached to your mobile phone, this is the best option in my opinion and significantly cheaper, just a little heavier.