Maybe you realize this already, but the shift kit for your 4T65E is little more than a few springs and spacers that "beef up" the pistons in the accumulator body. If you're going to drop the pan anyway to change out the filter and fluid, the shift kit is an extra 15-30 minutes work at most, as the accumulator body is right there. The ONLY watchout is to make sure the hard tranny lines to the accumulator are put back in place properly. Take digital pics as you go.
Just a note: When I shopped around for a shift kit, I settled on the Thrasher kit. For a few bucks more than TransGo, it came with a good Wix tranny filter, a GM accumulator body gasket, new accumulator piston seals, and a free T-shirt
2004 Buick Lesabre Limited, Gran Touring; Bone Stock
1998 Buick Lesabre Limited, Gran Touring; Mods: Poly sway bushings/endlinks, Tru-Cool trans cooler, Thrasher shift kit (med), PLIM, front PEM; Mods in progress: Alot of stuff, but slowly...