In all my experiences with B-spec and Super Speedway thus far, the B-spec driver simply doesn't do well in high-speed sweepers like that. I did the Super Speedway 150miles myself (using a Minolta Toyota 88C-V) and it was an easy win. However, in one of the race series, I was running a Formula GT against a field of mostly LeMans cars and Super Speedway was one of the races. So I drove the FGT myself until the first (and only) pit stop and handed the race over to the B-spec driver. He drove so freaking slow in the turns (on 5 mind you) that the Minolta 88C-V in 2nd came back from being down a lap to win within like 20 laps!
So when it came time to do the Formula GT World Championship, I found that the best option was to leave him on 5 the whole time, BUT make him take several extra laps after he first tries to pit until you get to the point where you can make fewer pit stops than the rest of the cars. His runs aren't even that much slower on red tires vs. green ones, but Super Speedway is the kind of track where an extra pit stop equals a ton of lost time, so ignore the lap times and just shoot for a couple fewer pit stops than the cpu drivers. (For example, if it's an 80 lap race and they pit on lap 10, then they will take 7 pit stops (10, 20, 30, 40, 50, 60, 70). However, if you stretch it out to 12 laps per pit for your B-spec driver, he can get by with only 6 stops (12, 24, 36, 48, 60, 72) and thus should take the win.)
This trick works in A-spec, too, as the time you gain with the better times is easily offset by the time lost by pitting on Super Speedway (especially with those walls you keep you from getting out of shape). In fact, even though I never used the "rubber band" trick in GT3 for Super Speedway 150 miles, I did run the whole race with no pitting and just destroyed the competition (and I used my 1881hp Escudo, IIRC...

).