A few tuners have said 250 bhp max on a standard bottom end and that's with careful choice of turbo and mapping.

St170 are a little stronger and should do your 270/280. Ive built one this way but ive not found any one to take it to the 350 mark that is reputed to be there limit.

Brexit hasn't changed anything, yet. And I am sure burton power will send you a set of Rs pistons.

Don't take this the wrong way but I bet ive got more Polish neighbor's than you do.
I would be amazed if you didn't know some one that could bring a set back with them and save the postage.

The Rs used a combination of blacktop and silvertop bits for the inlet. It will flow plenty for what you want. Some 440cc injectors will do the job fine.