That's a good question actually, regarding the ROM development. Past experience on the Desire Z/G2 phone was that ROMs can be developed to be agnostic of GSM network differences. Not being a developer, I'm just guessing that custom ROMs will be developed to leave the 3G/4G radio portion of the memory alone. But the issue of some models having 3G/4G and WiFi, and some WiFi only might complicate the matters.
Also, I don't know if it makes a difference that the UK Flyer is GSM (WCDMA?) and the Sprint EVO View is EV-DO and WiMax (from what little I know of Sprint's network). It might just be a difference in the radio software, or it might require completely different ROMs. I don't think we would know until people start messing around with the various stock ROMs and see what the difference is.
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Phone: HTC One X (AT&T version) Rooted, Bootloader Unlocked, S-Off
Tablet: Samsung Galaxy Note 8 (WiFi only) Rooted