I've also test driven a couple of V8 Sportwagons back when they were first available. I was pleasantly surprised at how quick they took off the mark. I would say from those 2 drives, a Commo will be quicker than a stock M35. Not by a huge margin but nevertheless will beat a Nissan. On the other hand, put a full aftermarket 3" exhaust and get rid of the extra 2 cats on a Stag and the performance difference will be marginal.
I was seriously considering buying a Sportswagon and could even live with the build quality (or lack of should I say), but the price tag did not justify the upgrade even though the dealer gave me quite a good trade in price at that time.