It'll be perfectly fine and I've used a Bigport flywheel on a 20v before on a stock ecu. Make sure you buy new flywheel bolts though as the auto ones are significantly shorter and will cause you problems if you use them on the manual flywheel.
The auto rev limit is 8200 , if you find a 6speed manual ecu the rev limit is 8400. I put a few youtube videos up a while ago where it videos me hitting limiter in my FWD 20v's. One is an ae101 and the other is the ae92.
The difference in flywheel weight is the removal of the excess material from outside of the clutch surface as shown in the picture below.
The weight difference can also be seen here