WELL,
I now have the 2 front doors done and the improvement is VERY good.
The hiss seems to be much less obvious, even given the MUCH higher quality tweeters installed.
As predicted the hiss is in part due to the crappy CAP's used to trim frequencies. HOWEVER, the improvement from simply replacing the standard speakers with a very decent set will result in some very good sound.
NOW: Given that I've now done a range of stereo mods to the factory 14 speaker MacIntosh setup, how would I grade the mods for effectiveness and improvement?
Mod -------> Result/Value
Deadening front & rear doors - excellent improvement. Flattens frequency response from top to bottom. Significant bass and mid range improvement. Lowers road noise as well, but not a huge amount.
Deadening parcel shelf - Very good improvement. Further bass improvement for the sub this time. Also VERY good road noise improvement over stock.
Speakers (rear) - Honestly, don't bother unless you're going to upgrade the head unit and amp to a much higher power rating, in which case you'll have most likely already done the speakers.

Anything less than a ~$400-500 spend here won't nett you a great improvement. The McIntosh speakers are quite reasonable considering.
Speakers (front doors) - Again, unless you're gonna spend ~750+ on a good set of 2 way speakers of a LOT more on a good set of 3 ways, then the factory speakers will be more than satisfactory for the vast majority of listeners. Yes, the depth isn't there, nor the midrange purity, nor the top end, nor the clarity in the bottom end BUT the frequency delivery is smooth and even from top to bottom, once doors & shelf are deadened there's PLENTY of bass for most people, from the factory, you could do a LOT worse (which Subaru historically have) this stereo is far from a standard Subaru stereo.
My picks at this stage:
1. Deaden the doors (double deaden the front doors, incl deflex pads)
2. Deaden the parcel shelf (complete single layer)
Total cost will be ~$300 and for the novice ~24man hours (complete disassembly, fit, reassembly).
I cannot stress enough how impressed I was just this mod on it's own.
The Dyn's take it to another level all together, but that's a VERY different goal to just very good basic music service at a super budget cost.