Max instanteous vTT is generally the same value as max instaneous vCore +- 5-10%. Intel doesn't recommend over 1.45V continuous vTT on 45Nm parts which is funnily enough the same soft limit for vCore continuous voltage. The buffer effect is why vTT should be soft limited to no more than the maximum safe vCore. A little over won't kill the chip as long as you compensate with a little less vCore