LLC is sort of a double-edged sword regardless of if it's on an Intel or AMD board. What LLC does is overvolt to attempt to compensate for droop under load. Depending on the board, it might overshoot quite a bit. As a consequence though, your CPU will definitely be more stable. Just be careful that it isn't stable because it's being fed 1.6V when you've set it to 1.525V.

Yes, overclocking the CPU NB will improve overall system performance. This frequency determines your L3 cache speed and also impacts your memory controller. 2.6~3.0 GHz is a pretty standard overclock range for this parameter. It defaults to 2 GHz on all models.

Don't worry about the HT link. Just keep it as close to 2GHz as you can for stability reasons. This is the link between the northbridge on the chipset and the CPU. Since there isn't exactly a lot of bandwidth needed, you're fine with whatever. Not hardly a benefit to risking stability for the sake of pumping it up high.

A CPU doing 4GHz at 1.34V stable is sort of rare, so I'd very much advise you to check what you're actually getting with a DMM. It might be delivering quite a bit more than you expect due to LLC. Also be mindful that it's definitely still possible that you'd crash after 20 hours if 1 hour of small FFT is alright. No platform is going to be able to save you from extensive testing. In any case though, I'm glad you're having fun. I'm curious what prompted you to jump onto a 1090T from a 980X. By all rights, the 980X is faster.