max safe volts always depend on the temps, not the cooling
thermaltake has watercooling gear as well you know but its far worse than most heatsinks hehe

id say 1.7v vtt and 1.7v vcore are probably fine 24/7 if you keep temps below 80C (of course you need chilled water or compressor cooling for such a high vcore with most chips )
if you want 100% completely safe then id say 1.5v vtt and 1.5v vcore at temps below 70C
at those settings you can probably run your rig for a decade without any problems...