John, sounds like we have nearly identical trucks. As my signiture shows, I have the 250 (#6 tst) plate installed... and love it. I was considering the 230 plate but was coersed into more power by EZ94 about 6 months or so back. I am glad I did go with more power. I had similar concerns as you about the transmission holding up. It shouldn't be a problem as you service your slush box, adjust the bands and change the fluid on a regular basis. I would also make sure your OD shift point is up there close to 50mph when applying power to save wear and tear on the transmission. If you have installed a 'mystery switch' or TC lock-up switch, don't turn it on and drop the hammer below 50mph or you just might hear some TC shudder. You don't want that. Better yet, don't apply it at all while accelerating.
Your mileage will vary depending on your need to feel the power of your bombed cummins. My mileage went down to 18. 5 but I had added a camper shell while testing the mileage, so it's a guess really. My advice is to check your transmission pan if you have 24,000 miles or more and do the service before you bomb. At least you will know all was well there before you start working it a little harder.
Good luck.
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98 qc, 12V, 4x4, slt laramie, 47re slushbox, 3:54 ls diff, 2YG pkg, Kelsey trailer brake control, dodge running boards, diamond plate bed rails, prime designs bug shields, silencer ring wind chime on porch, Raven cab-hi camper shell, psychotty air system, T/C lock-up switch, gutted cat, Walker muffler 22572, crome 5" tip from Kenworth stack, Isspro guages, A pillar 3 guage mount, TST #6 250/635.
Toys: 70' F250 4x4 highboy- 429cid 400+ hp on 35"BFG's(ranch truck) 64' MF 205 tractor 3cyl perkins diesel w/bucket