Lighter doesn't always result in faster. I'm a 2 handicap, and moved from DG300S to i95S (Steelfiber). I didn't find any considerable increase in speed due to the lighter shafts. I've tried lighter shafts and can't find my tempo with them. I also play heavier shafts in my wedges (S400). I found that I prefer the heavier shafts, but like the softer feel of the i95's on my older body (I'm 64). I also lost some consistency when trying out lighter shafts. I tried the i110's in SF, but the profile on that shaft is significantly different to the i95's.
My backup set of clubs has S300 shafts. I really like the feel of the heavier shaft. I'm thinking about trying the MMT105's for the slightly heavier shafts. On another point, I have made sure to match swing weights between the sets.
Good luck.