We had some views in our rails app that could be hit several times per second by our users, and they were uncacheable, so we implemented the views in c++ using libpg and fastcgi. So much awesomely faster.

If you've got some code that works well and you can solidify into c++ (ie - you're no longer tweaking it 3 times a day) - it's totally worth spending the time to rewrite.

I think C++ on rails would be a great idea, ie - some code generators to help you port your most used .rhtml views over to C++.