When I rode the 990, sport mode made it seem alot more like my 910: faster throttle response, less refined, etc. In fact, that's the best way I can describe it: regular mode was new brutale, sport mode was old.
From what I can tell the response at low speed is pretty marginal so you don't notice a great deal of difference, push it past 8000 revs and keep it in the higher rev range and sport mode really kicks in, more responsive and greater pull out of corners, drop the TC right down and your leaving black rubber lines out of the bends. Awesome!!!
I find the "normal" mode to be more mellow down low. Smoother, less jerky off the line as you accelerate. If you pin it it, still goes pretty good but I feel a flat spot at around 3k, then a surge. It's definitely a better map for city driving.