You can actually steady the framerate out fine even with that amount of garbage, just by carefully tuning the GC collection options. The only problem is that you a) can't do it under webstart and b) it's different for 1.4 and 1.5
Cas 
I only recently discovered this, whilst making an auto-jnlp-creator, but ... if webstart will honour the tuning settings, you can handle this: it has a built in feature of allowing you to specify multiple JVM's in parallel with *unique* JVM args to each.
Just kick your local Sun rep until they allow "ANY" JVM arg (anything you could type on the command line) as valid in webstart JNLP. There is no reason not to allow this, other than that sun thinks it isn't important. Off the top of myhead, 1.4 still only supports a tiny tiny subset (3 out of 300!) and 5 supports more, but still not the full 300?