Aaron, no doubt those are strong trannies, just spendy for what the average guy needs. You can replace the plastic thrust washers with bronze, they come in the thrust washer kit. You can buy six pinion planet sets that are machined to accept torrington bearings instead of washers, the tugger shift kit from transgo has the two snap rings that are prone to fail (coast clutch and overdrive pistons), you can install Red Eagle clutches and kolenes which increase the number of plates, you can install a 4 plate coast clutch drum, install a heavy duty intermediate sprag, one piece rear case bushing, PWM to ON/OFF tcc valve, a .427 boost valve, drill the drain back hole in the front pump cover, machine the OD planet where it rides in the hub and put a bushing in the hub, drill the cup plug .030" in the foward drum for lube oil, and yes, install the purple converter that BTS uses. (that is a Precision Industries unit, IMO the BEST anyone sells.) A billet OD hub and output shaft would be nice if you had 1000ft/lbs of torque, but normal street trucks pulling 5th wheels around dont need it.
I have built dozens upon dozens of 4r100 and E4OD's. The 4R100's already have steel planets, why upgrade if they are in good shape? I put a tugger kit, one piece bushing, red eagles clutches, HD intermediate sprag, PWM to ON/OFF valve, block the coast clutch valve on, drill the pump, replace the thrust washers, normal soft parts, a quality billet converter, a little of thisis and a little of thats, and not one has blown up yet. Rigs ranging from stock motors, to boom trucks, to rigs with stacked programs/propane/water and methanol injection, small 30 passenger buses, etc.
You can come hold my dick while a build a trans if you likie!
Basically, a few minor mods, a tugger kit, good converter and cooler even on stock clutches is a pretty darn strong 4R100 capable of pulling your 18klb fifth wheel anywhere you want to go, even with a strong program in the ECM
I did say, has not blown up yet, right?