I switched-out my Greenspeed Scorchers for my Primo Comets (same size) and thicker tubes last week when it started to rain (although the Scorcher is a nice, light and fast tire, I highly recommend that you DO NOT run them in the rain ). The Comets are 10gm heavier per tire than the Scorchers...not sure how much heavier the tubes are, but let's just say 10gm as well. I haven't noticed a huge loss in acceleration overall, but what is definitely noticeable is the added inertia. I'm riding fixed now, and brake primarily through backpedal force. I can really feel the difference in my legs when beginning my backpedal, to where I'm applying max force coming to a stop, and most definitely when going downhill. After about a dozen intersections, I feel my legs getting shakey in the backpedal, and find myself tapping on my brake a little to compensate. With the lighter Scorchers, I can go brakeless through my entire commute.

I can see now that for fixed riding, the less inertia, the better.