2C-I can be a bit rough physically at higher doses. Personally, the worst I experience is mild headaches and an odd ache/cramp in my right leg at higher doses, but I've heard reports of much worse reactions. Some people also find the stimulation a problem at higher doses, although I enjoy the stimulant effect of 2C-I and find it increasingly euphoric at higher doses.
As for intensity... Even at higher doses, 2C-I is in many ways a subtle experience. While it can certainly lend itself to intense experiences, the material does not seem predisposed to blast one's consciousness with heavy overt visuals or shatter the ego entirely. Visuals manifest in a unique way that is hard to describe; one minute I will look around the room and notice nothing unusual aside from brighter colors and a slight waviness or breathing effect, and the next I will catch myself staring at a wall as the whitewall texture becomes a chaotic swirl of moving patterns and colors. The more intense visual distortions seem to manifest either in peripheral vision, or only when looking at something in just the right way. The mental and physical effects are much more predictable than most psychedelics, in my experience so far at least. Overall, 2C-I seems much more suited for recreational tripping as opposed to trying to push the outer limits or achieve ego death.
The bottom line, of course, is that nobody can tell you how you will like higher doses of 2C-I. In my opinion, if you liked what you got at 15mg but want much more intensity, you may find higher doses satisfying. If what you got was not just too mild but fundamentally not the sort of trip you are seeking, then higher doses may be no better. I would try 20 or 25mg and work your way up from there. Be careful going much beyond 30mg, as others seem to start having unpleasant side effects beyond there. I've pushed it as far as 40mg so far and have yet to have a negative experience, although some of the higher dose trips would have been unpleasant without Advil to quell the discomfort on comedown.