This mod was done many many moons ago with the Gen 1 VR4's or TALON's in the US.
you crush the top, it preloads the spring, and therefore needs more boost to open.
However, crushing it also means that it doesn't vent as much at lower boost levels.
so you will get flutter at lower boost levels.
here is another mod you can do to them apart from crushing them.
http://www.thedodgegarage.com/turbo_bov_mod.html
There is also the "screw in the hole" mod that's been around.
if you want to see other mad dodgey mods and some decent ones..
go here..
http://vfaq.com/index-main.html
I think they even have instructions on how to build a little device you acn use to check for leaks in your intercooler piping.