I fail to see how the 86-87 cars will fair better than other C4's? I'll give you the minor geometry differences but gearing and weight?
At the MOST maybe 100lbs difference. Which is significant but I don't think its night and day.
Gearing, I don't think so, the best 86-87 cars can do is a 3.07 rear end. 89's could get 3.54 and many of the later cars are 3.45.
Even comparing 4+3 to ZF6 with the rear end differences: 1st gear (4+3)=8.84 (ZF6 with 3.45)=9.24 (These are overall ratios to the tire).
2nd Gear (4+3)=5.86 (ZF5 with 3.45)=6.21 So with the 3.54 it would be even worse. The later cars have shorter gearing. Advantage.
Even with an automatic (ALL ratios in the box are the same regardless of year or model of transmission) so with a 3.07 rear end 1st gear=9.39 2nd gear=5.00. Yes, you do get some from having a torque converter, granted.
IDK, about you guys but I run 2nd for auto-x almost all the time, only time I'm in 1st is for launch, then its 2nd gear all the time. Most SCCA courses top out in the low 60's. Which is perfect for a LT1, I'm right in the thick of the power band in 2nd gear 99% of the time and can rev enough to hit 65+ on the limiter in 2nd. By the numbers a 89 Z51 3.54 ZF6 car is THE C4 to have. Stiff suspension, big tires, good gearing and transmission, have to do with L98 but its a good engine, and its lighter than a 90 Z51 or the later Z07 cars.
Besides you guys are forgetting one VERY important item. TIRES, the biggest competitive street tire you can run on a 84-87 is a 255/40/17. (I would recommend the ZII over the Rival). Being as 88-later had 17" wheels they can run 18" wheels. I can put 285/30/18 under my 93 Z07. That is a huge difference in tire. The tire rule is +1 or -1 for your stock wheels 84-87 had stock 16", biggest you can go is 17"
IF C4's are competitive (which I doubt) it will be the 89 Z51 cars that have the best chance. Keep in mind they did well against the non-CR S2000's but that was on Hoosiers not street tires AND you have the C5 non-Z06 (FRC is LEGAL) to worry about. Granted the C5 cannot run as much tire due to 8.5" wide front wheel compared to 9.5" on the C4's. That being said the C5 has a WAY more advanced chassis and suspension design.
Just my 2 cents, take it for what its worth. I've been running my C4 on street tires from day one of auto-x with it. I've impressed more than a few folks on how fast a stock class C4 on street tires can go. By all means however, if you can run a 86-87 or hell any C4 and be competitive and win in B Street by all means do it!!!