Daz, put a pair on and use a MAP sensor ECU.
Do all the piping up (I've spoken to Jack about this BTW), then do a power run. Now, take the piping off and run just the new comp covers open to the air unfiltered and with zero additions to the comp covers. Another power run. See what happens - thats the only way to know for sure and yeah, isn't exactly a cheap thing to do
Edit; That may not make sense. Sorry, the only way to know for sure is to try some back to back, one set with the stock GTR size covers and the other set exactly as Garrett designed them