I used an OEM Brembo 18 x 19 for a few years and agree it works well (though, being an OEM piece, it is built to a price point). However, remember this unit was (first?) used with Brembo M4 calipers with 4 x 34mm pistons (on 1098/1198), where it worked very well. By using said master with 4 x 32mm pistons (as on GSX-S calipers), you will be sacrificing feel and power (less mechanical advantage). I ran this MC with Nissin calipers from an F4i (very robust caliper with 34/32mm pistons) and kept it when I switched to Nissin monoblocs from 2009 Daytona 675 (32/32mm pistons) and it did the job. I have since changed to a Galespeed forged MC (17.5 piston with 18mm fulcrum) this set-up is MUCH nicer. Choose your MC according to how you like your brakes to feel; if you like a firm lever (which, contrary to popular belief, actually generates LESS power) go towards a larger piston and/or longer fulcrum measurement. If, however, you like a softer lever (more travel but more power), then go towards a smaller piston and/or shorter fulcrum measurement. Again, do NOT get mislead into thinking a stiffer lever (like that provided by a 19x20 MC, for example) will generate more power for the opposite is true (it has less mechanical advantage). For a 32/32mm caliper, my personal choice would be for a Brembo 19x16 (only in billet; expensive but nice) or something 17 (actual 17.5mm) with 18mm fulcrum (such as the Nissin unit used on select GSX-Rs and CBR-RRs - that being said, I am not sure what comes stock on GSX-S, sorry, so perhaps I am being redundant). You can also ante up and get a Brembo RCS17 (allows you to adjust the fulcrum length from 18 to 20mm) or try a Galespeed (reasonably priced for a top-notch item). FWIW, I have tried numerous caliper and MC combinations over the years (with my VTR1000 projet; thanks to OCMD - Obsessive Compulsive Modding Disorder) and find the best performance to be with a set-up which generates a lot of mechanical advantage. While requiring slightly more lever travel, it gives more power and, most importantly, better modulation and feedback.
Hope this doesn't come across as patronizing for it is not my intention.