The problem with this line of reasoning is that a large amount of cache will often not give you much more benefit than a small amount. Indeed, that's the nature of caching: you get most of the benefit from the first bit of cache, but the level of added benefit drops dramatically with more cache.
What if using 400MB of cache for FCP only gave 5% of a net performance advantage over using 40MB of cache? Would it still be worth it to take away that extra 360MB from Safari?
And there's the issue of human psychology: people deal much more easily with a little slowdown spread evenly than with a full-on stop for a short amount of time (even if the full-on stop scenario gives you greater average performance). I'd prefer Aperture run 5% more slowly than it might otherwise, if that meant I never saw a beachball when running Safari.