Shmidt Bros Motorsports makes a few upgrades that affect handling that I have done on my sled that have made a massive improvement. Sloppy Steering fix - replaces the shim stack at the collector tightening it right up. A-arm shims- cheap and effective, moves the bottom a-arm mount position increasing camber and limiting reducing wear from excessive lower a-arm movement. Steering tie-rod bolts- really micky mouse but the factory bolts do allow slop at the mount, changing to rolled bolts offer a tighter tolerance at the mount. Case in point there is little bits of slop inherently in the Phazer, these fixes take care of a few of them. I would also recommend Slydog skiis, decent carbides, sway-bar and suspension setup will all affect handling.
For exhaust, I had MBRP but hated it-nothing but noise IMO. Sent it to Schmidt bros to have it rebuilt into one of theirs-highly recommended with airbox filter kit.