I think you may be being a little harsh on Tim and the team.
I have tried various 5" devices with SD. I found the worst readability with a high resolution (800X480) cheapish Chinese device .. resistive screen. By far the best is still Navigon 8410 (but three or four times the price) .. its only 480X272 but capacitive, high contrast 400 NIT screen. In between but still pretty good is cheapo Chinese (£50 ish).
I also have a 7" device (800X480) and only when screen is this large does the higher resolution work for me. I also use a 7" UMPC running at 1024X600 .. fine on the ground for flight planning but probably worst of the lot in the cockpit.
Just my own personal preferences of course but as said a good deal elsewhere readability is a combination of lots of things .. resolution certainly but also contrast, reflectiveness, capacitive vs resistive, coating .. among others. Since its almost impossible to get an ideal device made in small numbers at ANY price, we are all stuck with the specs that reign for car navigation using WinCE.
From there we all have to make our personal trade-offs.
My personal favourite from all this lot? Navsgo 7100 .. 7", 480X800, 400 NITS, resistive BUT ..... low memory, slow processor & no graphics acceleration .. cost about £150 delivered UK but now superceded by 7200 with slightly lower screen brightness!!
Cheers
D