I have been thinking of this issue a lot lately. I was actually going to start a thread in the T-Cell, but I guess we can keep the discussion here for now. I haven’t come to a firm enough conclusion to adequately answer your question, but I’ll throw out some of my opinions.
First of all though, it is very important to consider your body fat when discussing this topic. It goes without saying that your athletic performance would suffer if the extra 15-20 lbs were all fat. It can be very difficult to gauge your own bf%, so it is tough to be honest with yourself on this topic. In your case (at 6’3"), I find it hard to believe that your athleticism goes down from 195-210 if your bf stays the same.
This leads into another point. When gaining muscle, fat is frequently also gained. The muscle:fat gain ratio depends on a number of variables, but one thing for sure is that the more muscle you have gained over the years, the more fat you will gain when adding additional muscle. I think you have to have gained quite a substantial amount of muscle before the muscle becomes a hinderance, but the added fat will quickly slow you down. So in the case of an athlete (exluding sports and positions where any weight is good) I firmly believe a clean bulk is far and away the best route.
In other words, it’s not fair to compare your athletic performance at 210 lbs and 15% bf with it at 190 lbs and 10% bf.
You also asked for personal experience, and my experience is why I have been thinking about this lately. I’m 5’10" and almost certainly would not weigh over 160 lbs had I never lifted weights or actively tried to gain weight. At one point I thought I was my most athletic at 185 lbs. When I got relatively lean at 195 I realized I was wrong.
I’m at 210 today, and my endurance, speed, and athletic ability is definitely suffering. The last 10 lbs has probably been half fat. Part of me wants to hit 220 before cutting back to 8%, but part of me is ready to lean up now. Sprinting has always been a passion of mine, and I would like to start competing in amateur meets again. Even at 195, a 400m is out of the question. However, with my added strength, I am much faster and stronger than I have ever been, and if I had my endurance back I could still run a pretty respectable 200m.
The other issue I have is that it is extremely hard to maintain muscle and remain flexible. I can just about gaurantee that I would hurt myself if I went out to the track and ran a dead sprint. It is an akward situation knowing that I am stronger than I have ever been, but I am unable to use the strength in any sort of speed-strength situation due to the near certainty of injury. To get back in the shape where I can go out and run without worrying about injury would definitely require sacrificing some muscle mass.