I have recently “discovered” WP-Minify, Thaya Kareeson's excellent plugin port to WP of the Minify project.
Now, unifying (combining) and minifying CSS and JS for Simple:Press Forum is not exactly possible, so I've added the string “simple-forum” (no quotes) to the exception list of the WP-Mnify settings, to leave those completely alone for SPF, similar to excluding the /simple-forum/ directory from consideration by WP Super Cache.
But that's a pity, really.
So, would it be possible to consider some support for it? Perhaps one more straightforward way – a long shot, not exactly obvious or “user-friendly” – might be to create a clone of the header.php template file, and call stylesheets there so as to minify them separately for the (also separate) template page file on which SPF lives… Dunno.
But given the myriad of CSS and JS calls going on in SPF, it might be worthwhile to consider it as a way to alleviate servers. As an aside bonus of sorts, of that “separate header template” method, one could also include the necessary browser cache (ETags, expires etc.) instructions to further reduce load, as nothing beats fishing static elements from the user's browser cache, server efficiency-wise.
Just an idea for some imaginary dull moment in a stretch (way) further down the road!