The answer for the first question is… Yes and No.
Modified firmwares sometimes use a better stratage, so there is a better chance that the output disc is readable in your NEC2500A. But modified firmware itself does not increase readability of a given disc. (At least in my experience.)
My G04 can be written at 8x, but have some chance of having “readability problem”.(Still reads good in other dirves). At 4x it all works fine even in 2500A.
Well… if you pick up a good media, maybe you can write it at 8x and fully read it back. But if you do care the quality of discs, maybe it’s a good idea to make discs that 2500A can read. (In that sense, 2500A is a good indicator if a disc is good. Anything readable in 2500A should be fine at all drives. )
Some people say their 2500A is quite good as a reader. I’m not sure if they’re just using a better media, or… maybe their 2500A is really a better reader?
But nobody seem to have compared “good” and “bad” 2500As.