I've got a 512MB cachecard in my series 1. It speeds up DB access, but doesn't alter UI performance. For most people the latency is in DB access (which you've described as UI speed), but I still find the UI itself sluggish when not hitting the DB. We were talking about two different speed measures.

The DB issue could also be improved with a better DB scheme I believe - but given the resource constraints in the box, it will never be lightening fast (even with a cahcecard). We done some tests with enormous amounts of content using SQLite and speed was nothing short of phenomenal even with modest amounts of system memory and definitely without a dedicated 512MB cache. But, big differences exist between our test platform and a TiVo box.

Bruno