Let's re-iterate.... No.

If you think about it, even with the tensioner in place, the chain is still passing across the same amount of teeth on the rear cog in order to drive the rear wheel. Thus, your gear/inch ratio remains the same. You'll just be running a longer chain, which is probably adding at least 10 grams of rotational weight! Best sort it out and take a hacksaw to the rear dropouts so you can fit some track drops.