Which is more tuneable the UCE
Or the carbed pre UCE engine.
For the carbed version up rated carbs are available to
improve the fuel/air ratio after an exhaust and filter is added
How is the EFI changable to suite these bolt ons
EFI can be tuned with a Power Commander or other similar ECU modification and a computer with the proper software.
If you want power upgrades, the old Iron Barrel 500/535 can produce more power than any other kind of Enfield single, if it is properly modified. However, the old ones require the most money put into them, because the bottom end can't handle any extra power unless heavily modified.
The carburetor is not real high on the scale of importance in this whole scheme of things. There are many other things that are a lot more important. You could change the carb to a bigger one, and gain nothing, or even lose power.