Yeah I was pretty pleased with my sub 25*c deltas on the G3258 lol. But once you go 4 cores these things are hot so I'd be happy with 45*c IHS/cold plate temp to core temp delta. Really I only aim to drop my temps about ~20*c with TEC cooling so that I can push an extra 100-200mhz over water alone, any further drop below that is just a bonus. With a TEC chiller I try to get that to 30-40*c temp drop to be worth the cost of the setup. Beyond that, phase change chill box territory.
I contacted swiftech directly and they can sell me a revision 2 XT top for USD$21+shipping, a new Apogee XL block was USD$64+shipping and since I can order direct from them I can buy that instead of modifying my XT block and wasting money on the revision 2 top that I may not use. I'll sell my XT to my brother with my spare 240 rad so he can start a water cooling loop on his 5820k, I bought 3 EK 3000rpm Furious Vardars for my Hardware Labs Alpha Extreme III rad and a Gentle Typhoon AP29 3000rpm for my EK Coolstream XT 120 rad (I just wanted to compare the vardars to the typhoons really) but basically it means I don't need that 240 rad in my loop any more as it was just getting too crowded in my case, plus my ambient noise in my apartment in the city is so loud I can't even hear the fans lol, so I'll let them crank right up for the lowest possible deltas, I'll still be adding the 2 12715 tecs in at 5v on my GPU blocks in the hot loop to chill those last few degrees to ambient (if it works). As my new TEC is 40x35mm I think the PIN matrix on the XL is just about the right size for total coverage of the TEC and the ports are in the correct locations for even cooling of the whole TEC and the restriction should be less as well.
Here is a pic of the XL baseplate again. I'll remove the centre o-ring, no mods required other than that. I'll take the mount bolts out and use my setup with the M3 bolts threaded through from the back plate and tightened down with nuts.