it will affect it A LOT. most ddr2 that is spec'd at 2t will not run 1T over its stock speed. ie if you have ddr2-800 spec'd at 2T, it will likely not run 1T at any frequency higher than 400mhz. even close to that is a challenge most of the time. for it to make a difference, you need to keep the other timings as tight as possible.
like, if you go to CAS 6 with 1T, you might as well stay at CAS 4 with 2T.