I would love to see an LT5 in a "Z". My concern though is: How much support is there for that motor? It may be cheaper to wring horsepower out of an LS1 and parts are available everywhere. Of course it would be much cooler to run an LT5, kind of like the Jag V-12 one fellow on this list swapped into his Z.
Also the initial cost looks quite high. Car-part.com shows wrecking yards offering LT5 motors starting at about $10,000 and running up to $16,000. If I had that kind of money I think I'd buy a seventies or eighties Porsche 911 and sink $$$ into that.