There have been several threads posted on the web regarding the virtues and drawbacks of fiberglass panels, etc.
I chose to use steel flares from a nice SC on last year's RS replica. That single fact made the restoration go
relatively easy (from a fit/finish point of view) and the finished product turned out great at least IMHO.
The bumper ends met and all was well. I did use fiberglass bumpers and wing on that project, which I am still happy about.
This one is another matter altogether. So, some advice for other RSR enthusiasts:

Use steel rear flares if you can find them and make sure they are truly designed for 15"x11" rims.
The 11" glass ones in the photos were supposed to accomodate 11" rims and might have covered 285/40/15 tires,
but absolutely would not handle 345's. On the subject of tires, I noted a particularly well done
RSR in the July, 2000 issue of 911 & Porsche World magazine. This car had Yokohama 285/40/15'
AVSi's on the rear and 225's on the front on 15x11" and 15x9" Fuchs respectively.

Only Yokohama and Pirelli make tires approaching these dimensions in 15" today and neither
currently offer the 285 width, so my second key piece of advice for you is that unless one of the
tire manufacturers brings back the 285's, you'll have to go with 345's on the rear as I did.
The rims certainly accomodate them fine, but even the 11" flares I obtained came nowhere close to
covering the tires. They were in fact shy by an inch. Upon test fitting the rear bumper, I noted
that the ends (where the flares met it) were about an inch wider than the body including the new
11" fiberglass flares.

Do check these things out in advance if you can...

Jack McAllister