I would stick with serial...It's a tried-and-true setup for the vast majority of 911s that are running extra coolers. I think it's also probably moot, considering a center-mounted cooler will get waaay more airflow than a fender cooler. I suspect that would render the difference negligible...and a good fan might solve a lot of problems anyway. Running tubes places where they were never expected...Regulating even pressure/flow between two paths...Maybe stressing the oil pump more (?)...I dunno. I think you've nailed the pros and cons, and my choice would be clear.
When my 250-300hp motor (3.6 out of a 993) had a center cooler, it never even hinted at exceeding 210°F. Now that it only has one fender cooler (with a nasty fan), I've only seen it go as high as 230°F on a warm sunny day going up a mountain. I think two fender coolers with monster fans would be more than adequate for a Subaru motor (pure conjecture...).
One thing is clear to me: a 912 with a stock look and an STi motor would be sweet. I've often fantasized about
putting a Subaru transmission in as well...