Actually that was my mistake. The max actually for the Roadster is 4.15V (so 410.85 for the pack), not 4.2V. This is to protect the long term battery life. I think you see 4.15V after a range charge (100%). 4.1V@85% seems normal.
http://www.teslamotors.com/blog/bit-about-batteries
I don't know if the Roadster interface still allows you to see battery voltage after this, but you may have to disconnect the charger, maybe use some light accessories or drive it a bit to see how the battery voltage responds. Immediately off the charger it might hold to a high voltage. If at about 50% SOC your cell voltage is still at about 3.7V (with a light load) then it should not have anything to do with your battery age, but maybe more with the fact that it's a 1.5.
But overall, I don't think it's much to worry about. From the looks of it a brand new 1.5 probably got something slightly over 4.0 seconds (I think Road and Track got this number in their test of a 2009 Roadster with a 1 ft rollout). And given the error range of your measurement device is about 0.2 seconds (seems kind of high, I'm assuming maybe +/- 0.1 seconds) it seems your car might be on the dot. I think the main thing that matters for the G-Tech is a flat road and that it measures assuming absolutely no rollout. Road and Track says the rollout is subtracted from their time and this can vary from 0.2 seconds to 0.4 seconds, so the real standing 0-60 time they got is probably ~4.2 seconds.
http://www.roadandtrack.com/car-reviews/2009-tesla-roadster
http://www.roadandtrack.com/special-reports/how-we-test