Hi Andrew,
Don't think I've had any firsthand experience with 5EFHE pistons in a 4E before, I've guessing you already have the head off so can give us a few pics (just out of curiosity more than anything)?
Quick summary of the ratios seen across the range:
4EFE runs 9.6 CR
4EFTE runs 8.2 CR
5EFE runs 9.4 CR
5EFHE runs 9.8 CR
This means that it's a fair step up in terms of compression from your 4EFTE setup. As you already know this could limit your boost level somewhat as she will have a higher risk of detonation/knock. No surprise you are looking at a shim plate. I've heard of people fitting two headgaskets to acquire the same effect but that's a bit too 'backyard'
Definitely worth dropping the sump off to confirm which set of rods you have - the thinner flavor doesn't have a good reputation on low compression engines never mind high compression ones, could be another limiting factor..
On the good news front I've turbo'd two potential detonator 4EFE's with encouraging results. Neither high boost but they did function and last without exploding. A gentle tune and a careful ear for detonation might just pay off!