Have a look from start of [Only registered and activated users can see links. ] for a couple of pages re different gps hw & fw compatibility. Distilling it down to layman's level [mine]i think that the 2 different gps chips, TI & MTK, are the reason that there is a wide divergence in accuracy. One of them is simply more compatible with 34x0 gps fw i think.
How would you know in advance would be what you want to know i suppose. Unless you know which is the problem chip [we don't afaik] as well as which one you have ... then, you don't know. Just flash to 3490 and see what accuracy you get. I've flashed back and forth and accuracy returns to the reported '3 metres' as 3790 everytime (as 3490 i get 4-10 metres everytime, still acceptable).
If you convert let us know the accuracy.