Rear cogs are easier/cheaper to change, unless you already have the parts on hand. I guess you are using a tensioner.
BMC??? sounds sweet! post a photo.
If you really want to develop, why not get a new rear wheel with a track hub spaced to 130mm so you can go fixed. It's best to not use a tensioner for fixed cogs, so if you build the rear wheel with a White industrys Eno eccentric hub, you can get good tension with vertical dropouts.
Fixed is good for training, because it forces you to develop you spinning technique, which it seems (via consensus on the matter) makes you a faster rider. Isn't that why Lance trains with a FG???
SS is good, but downhill you can just coast, which doesn't help the training much I am sure.