Another option regarding the GPS is to use the Topeak Bar-Xtender. I have trekking bars on my Fargo and the Arkel bag's brackets don't leave a lot of uncurved bar for gadgets/lights so I picked up a Topeak Bar X-Tender and it works great at any angle.
Also, regarding the Arkle -- my wife and I each have the large bag and, in hindsight, I think we'd both prefer the smaller. The bag's contents stay dry, it doesn't budge at all even over rougher roads and bumps, but we are constantly reminding ourselves to NOT pack it too full. It's size is great for wallet, Kindle, snacks... then we add a spare pair of gloves or our bunched up lightweight windbreaker to keep things from jostling. It's a great bag, but I'd be just as happy if it was about 1n inch smaller in all dimensions.