It'd be a laggy beast. I've got a T3/O4E with .57 trim, and while it's capable of pushing tons and tons and tons of boost, the T3 turbine and ceramic bearing keep it from being terribly lagged. Even so... its noticable. The T04's are really meant for large displacement cars. If you're intent on pushing more than 15 pounds of boost, look into the T3/04 hybrids, or even at a T3. The trick isn't to get the biggest turbo you can, its to get the smallest turbo you can run at max efficiency at your max boost. In other words, the one with the least amount of lag that will be operating at its best, when you're calling for all it's got. Talk to someone at Turbonetics, Garrett, or even a really (stress REALLY) reputable, knowledgable shop... they'll be able to sit down with your engine specs and match them up to turbine chart, that will determine whats best for you.
Just a sidenote to your other questions, I was running 6 pounds of boost on stock internals until something went haywire and caused some part of my engine to start clacking. The jury's still out on what happened and why.