I have tested most things to do with CON stat wise and skill wise(Guessing its similar with THM, not 50 yet on THM). From personal experience, INT damage does increase, but as you increase it the effect gets lower and lower as it progresses through the Tiers(Which I am not sure at what point that occur, may be when it takes 1 more point to increase it a rank). As far as PIE goes, that skills does make a difference. As a previous post stated, they do increase DoT time on mobs. For me the most important thing about PIE was by the higher the PIE is the more Siphon MP you could do. When I left mine at 30 for example I could not siphon past 50 on dead or alive mobs, while with 100 PIE I siphon 140 most the time off dead mobs. Also resisting mobs spells seems to occur more. So if I had to take a preference only put INT up a little, but I would put up PIE to at least 100 as well as MND to 100.