The only thing you should upgrade is the center diff. The tcase will last forever if you arent running a solid center diff (welded diff, spool, welded VC or VC Eliminator, etc). The front and rear diffs are strong. And to be honest people have run stock center diffs well into the 11s. The key point to keep in mind is wheelhop. I destroyed a center diff on the t25 running mid 14s
So if you dont wheel hop you are usually fine. The risk you are taking though is this. If the diff does break, pieces of it may get out and dstroy the rest of the tranny. In my case I was lucky, it broke into big pieces that stayed inside the center diff case. My friend (Joe G on POSR) wasnt so fortunate. His broke, and wreaked havoc inside the rest of the tranny before he hit the end of the track. I believe it was a 2100 dollar buildup by sheppard, thanks to all the damage, while a typical shep rebuild might be closer to 1400 with the same upgrades. So if that happens, it hurts the wallet. The choice is yours. Avoid wheelhop like the plague...