I wouldn't recommend that.

First of all, increasing caster won't decrease understeer. It may for a fraction of a second, but only because it causes less stability, so as to make the reaction of the turn faster. Thus, it improves turn in, but not understeer on a whole.

If you really want to decrease understeer, do the regular bolt ons i.e. sway bars, struts/shocks, springs, bushings, strut tower brace. Yes, I know they are expensive, but if you do the work yourself, you won't believe how cheap it becomes. And then there's the fact that it's the only proper way to do it.

Also, get better tires. I know for a fact that the Yoko A520 or Paradas you are running, are nowhere near the better performance tires out there like the Bridgestone SO-2, SO-3, Toyo Proxes T1-S, etc. Simply switching from those Yoko's to one of these tires will get you on average, about .02 on the 200 ft. skid pad.