Training the higher levels of any skills has low chances for success even if your gladiator has 200 Intellect. There will be some improvement overall but it will be something like 15% for training one point from 9 to 10 instead of 10-11% with a significantly dumber gladiator.

In general, Intellect helps your gladiator perform better and is integrated in the efficiency calculation of many skills but you should not stack it like, say, Strength, Agility of Stamina because you'll end up with a slow, fragile and weak smartass. I'd say 100-110 Intellect on a lvl 55 fighter in Blood Gods is enough for almost all builds.