i found with my modded primary turbo i needed significant tweaks to wastegate duty cycle. i did this in the tune, but you could use a manual boost controller instead.
i looked at using the vf27 in the ported exhaust housing that i have, and while the centre fits the housing, the same doesn't work the other way - the vf31/32 centres don't fit the vf25/27 exhaust housings. i never ended up trying it, i figured the extra space would create more lag, but who knows?
pretty much given up on the TT now and going single again soon. while the ported turbos and ecv do help, nothing gets away from the changeover, and worse, how long it takes the ECV to close again, which is a real problem if you want to shift and end up just on the primary again. it's either 4000rpm or 6500-7000rpm.