See topic.
I got both Titan Quest and the expansion already, but there are MASSIVE drops in fps in the levels just before the cerberus fight, and when fighting Hades himself. On all machines I tried it on, AMD/Nvidia cards, AMD/Intel CPUs....it doesn't matter.

Are those issues fixed in this version ?

Yup: "Fixed lags caused by Tower of Judgement and Crystaline Stellae scripts"

Awesome, guess I'll have to buy it.

I suffer random massive lag everywhere. Ten minutes of gaming and the game lags a lot.
I remember running the original+expansion very smooth.