"i find it hard to believe that the power demand in the Kanto area would have not been met more successfully if there had been more transmission capacity between the various utilities."

Yes, of course, Luta-san. No need for numbercrunching here, since it's a matter of logic, not arithemetic.
But wishing away the 50-60 Hz divide is like wishing that your aunt had wheels. (So, so much for Kyushu and
Shikoku.) Actually, they've begun working on it, but it's very expensive, which is why there was so little effort being made for something whose utility was highly limited--till now. (But anyone who foresaw the disaster and its consequences on electricity supply against the social and poltical backdrop is talking through his/her hat.)

As for the interconnectivity between EPCOs within the same superregional grid, last summer, according to media reports, the maximum transfer capacity between Tokyo and Tohoku was 3.31GW; Tohoku wound up taking up 1.7GW at the peak. The Hokkaido link was much smaller, while Hokuriki had little reserve capacity to spare.
Does that mean that Hokkaido could have sold a GW or two more to Tohoku if there had been a bigger link to the mainland? Perhaps. And would that have had a positive kncok-on effect on Tokyo? But who was going to build that? Someone would have had to pay for a normally little-used underwater, high-voltage transmission line, and try telling that to your shareholders.

Of course a more deeply interconnected national grid would, other things being the same, have meant less generation capacity nationwide because of the lower need for redundancy. Given the nationwide drop-ffin nuclear power generation, I'm not sure at all that we would have been better off on electricity supply in that case. (To be sure, I'm personally in favor of more separation between generation and transmission, though I'm not sure just how to go about doing it and what to make of distribution. But that's a different issue and I wanted to keep my comments free of complications.)

Have a nice weekend, everyone!