There's a huge number of variables that affect how well your car will perform at the track. The ones that are running the big numbers have all these variables (or a least a lot of them) worked out to provide them with the optimum results.
FWIW stock motored stangs can and have run well into the 12's with weight reduction/suspension and good driving. remember, even stock motors have a large variance in performance. every motor must be dyno/track tuned to know optimum shift points etc.
A while back, someone on here was joking around when they said, "What does a 300HP, 400HP, and 600HP Supra have in common?...A 12 sec. time slip." The point being, there comes a point where increasing engine power no longer translates into improved ETs due to the lack of traction. Certainly, we can all agree that as the class of car increases from say, Factory Stock, to Pure Street, to Drag Radial, to Pro 5.0, there are dramatic differences in chassis, tires, and suspension to deal with increasing engine power.
One thing I did learn from the pros at Alternative Auto and Detroit Speedworks is that your typical 600HP Mustang will run 11s and 12s on street tires, but can run 9s with a tuned suspension and slicks.
Another excellent example is to look at the NMRA Factory Stock class. A relatively heavy car, relatively under powered, and nailing down high 11s and low 12s. Most are dedicated drag cars, not something that can be driven back and forth to work and school.
Most hobbiests that I see on here have the power, but are still running a "street friendly" suspension. That's fine and dandy, but don't expect to nail a 1.3 sec. 60' time with it...even with slicks. Most people that I've seen posting their time slips have a difficult time breaking the 2.0 sec 60' barrier. Most can't just drop the clutch at 4-5K and expect the tires to grab, most of the time, it's pedalling it from around 2K with a street suspension. In addition, most folks will tell you that the 60' time is one of the most important indicators of a great ET. It is very difficult to overcome a poor launch.