If you really want to stretch out the .357 Magnum, you might try finding loads for the 200gr Hornady Spire Point. There are a few guys who run these bullets at our local silhouette matches out of the .357 Maximums, both Dan Wessons and Rugers. The GP100 has a long cylinder to begin with, (I believe it can fit the 360DW round), so deepening the chamber to accommodate the longer bullet while also utilizing a new barrel with a twist-rate appropriate to the longer bullet, might get you some interesting results at longer ranges.
