Are LED brake lights just a bulb replacement solution? Has anybody on this board actually done this with a result that solves the problem?The dimming effect you have observed is ordinary and common to quite a few Subarus...
If it bothers you to the point where you want to do something about it, I might suggest converting to LED brake lights- the reduced current draw will make for less of a voltage drop.
I haven't seen an aftermarket voltage regulator.
For later models, you could consider using freeSSM to artificially raise the idle.
Is the dimming also related to the ABS, Battery dash lights coming on intermittently?