Thank you for your reply
I've searched a few more threads and come up with that Berk mids will dampen the sound rather than increase it without DPs. I will fit DPs some day, maybe but that's a way off yet so N55 mids looks like my mid-term solution.
Prices for these are pretty low these days so I guess I can take a gamble and like you you say, fit first without deleting the resonator and if I want more then cut it off. I guess the beauty with the BMWPE Y-pipe is the fact that the resonator can be reattached with clamps when ever you want.
I've no doubt that if I ever decide to install DPs I'll need something like Berk Mids to tame it a little but for now I'll have some fun with this.