The problem is most DP's were designed to replace the stock US-spec pre-cats. If you go with aftermarket DP's you need TT midpipes and your right back to where you started. The only way to make the 2+2 pipes fit is with j-spec DP's. I have a set of J-Spec's here that might be up for sale soon, but they shouldn't be to hard to track down.

Additional pointless info:
The precats only came on US-spec TT's, so the TT's got shorter midpipes to make everything fit right.
In Japan they didn't use precats so all Z's have the same midpipes.