I changed to Diablo Rosso Corsa 3 tires and have been very happy with them. The change I did was to use a 190/55 rear profile. Between 40 to 110 degrees. I do Street and track days and very little rain. The grip is phenomenal for those conditions. I cannot attest to conditions outside of that
S21 and Diablo Corsa III were my 2 choices in the end. I was looking for the sportiest thing with highest dry grip that was adapted for the streets, while offering decent/safe wet grip (I barely drive in the rain and would always always drive slow in the rain). Lifespan of 7-10k km is good enough for me. Any better is great, as long as dry grip is not sacrificed. $$$ was not a real factor. I went with the Pirelli, I just got them actually. So far so good, but it's too early to tell. I'm pretty happy I picked the Rosso III so far to say the truth, my first set of Pirellis. My dad kept telling he had Pirellis before and found them slippery. I hope to prove him wrong, and I think I will be able to. The only downside I can think of right now is they are pricier than most brands, including the also very good (according to reviews) S21.
From what i understand, S21 will give you better mileage and save you a few $$ while Rosso III will give you better dry grip/handling. But the 2 appear to be up there anyway. My feeling is that I couldn't go wrong by picking either. I didn't really want the extra milage this time since I hope the replace the 2 tires simultaneously next time.
I have had an Angel GT on the rear for the last 5000 miles. I really like it and will get another one when it is done. I figure it has another 1500 miles left in it or more.
As for the front I have a Diablo Rosso II. It seems to be wearing much faster than the Angel on the rear but it really feels great and gives me confidence.
I switched to Pirellis when the oem tires wore out. I was familiar with them since my other bike, Ducati Diavel, came with them from the factory. For the GsxsF I will go with Rosso III on front and Angel GT on rear when the time comes.
I have just fitted roadsmart II to the GSXs ( i do like these tyres ) and after killing a set of Bridgestone BT016 at the track the other day on the GSXR i have replaced them with Pirelli Angel Gt so will report back on them. Have had Pirelli diablo rosso on the fireblade in the past and once warmed up, loved them, loads of grip and confidence inspiring but crap in the wet.