I guess nothing as long as you have a small turbo that produces good power low end, but they're hardpressed to find one that produces solid power and torque numbers below say, 3500rpm. As long as you can keep the intake temps down. 19psi is alot of heat on a smaller turbo. Im not very familiar with many different types of turbo's tho, so I could be misinformed.

Ive seen t25's pushing out 22psi. I do not know thier efficiency at such boost levels. I can't imagine it would be great. Although a t25 might be hard to get to spool to... 8k rpm like Honda motors love to do. As long as it's not so high-reving, the turbo wouldn't get out-spooled(I don't recall the term). Peaking at near or over it's efficiency RPM, it would take a nice cool day, methanol injection and a WELL matched intercooler. Not to mention a good tuner.

Again, these are just my limited knowledge theories on the matter. I have very little expierence with engines and thier dynamic, even less with boosted engines. Hope Ive been of some help. Maybe someone else could pop in and school me on what im wrong about.

As far as your question. You know the answer and what would have to be over come.
Detonation, Aluminum block, Fuel, and Tuning. Sleeves would probably be a must for that much boost on a Honda block. Altho I believe Mike and Chris said they've run 19psi on that LS motor. (With race gas though.) I might not be remembering correctly. So don't quote me.