From my observation, the performance difference between 1T and 2T isn't as big as in 680i. 680i performed meaningfully better with 1T than with 2T, albeit mostly noticed in benchmarks.

It looks like achieving 1T is easier with 780i, but the performance improvement isn't significant. I'm guessing that NV loosened the internals for better compatibility. I don't think it matters a lot since current trend is going towards capacity than performance, when it comes to DDR2.