I have run my SLC a lot with GT3 Porsches. All in all my best estimate is a 450hp SLC on R7 Hoosiers can run very similar lap times IF the drivers are comparable in skill. The one big difference is I have a G50 5-speed GRRX with a Quiafe TBD in it and I have to row through the gearbox myself. Coming off a slow corner onto the longest straight I am in 3rd and by the time I have gotten to 5th the Porsche has shifted 4 times with his PDK and pulled me by 2-4 car lengths. A GT3 Porsche has about the same power and really isn't much heavier if at all. It's the ability to make instant shifts.
At the end of the back straight we are right at 145-150. I can brake with him and maybe even get my couple of car lengths back, but it is clear I need to do 2 perfect downshifts back down to 3rd, get the breaking right, and turn the car in at the right spot. Coming off that corner we will be equal more or less because we will stay in third for two more corners, But the next time we need to make 2 or 3 upshifts the same thing happens. I will inevitably make a slow shift going up or down and if I do I can't get that time back. In 8 laps he's gone.
I intend to fix this with another 125 HP. We'll see how that goes.
But it will always come down to the driver in near-pear cars.
So will a GT40 kit car go as fast as a GT3, Yes. How?
1. 400 inch small block @ 500-550HP
2. 18-10F and 18-12 rear wheels running dot slicks
3. Correctly designed suspension geometry suited for the tires
4. 14 inch rotors in front and 12 in rear with appropriate calipers. All vented correctly. I don't think you need antilock brakes. BUT you will need to two-foot the car in the braking zone..................WELL.
5. Keep the car down to 2500 pounds or as close as possible. Leave ALL the bling off! Figure 10 gal fuel tank. You will use about 6-7 gals every 30-35 mins.
6. You will need to spend a lot of money on a modern racecar transaxle that can be shifted electronically and match downshift revs automatically with the engine management computer.
7. The car will need to run an efficient splitter and dive planes on the front and a large spoiler on the rear. Flat bottom the car as much as possible while running the car in a front-down attitude rake. I don't think you will need a wing. The total downforce on the front of the car can be balanced with a spoiler at the rear.
How much? If you don't pay anyone to do anything for you so the labor costs are near zero, you will have about $50K in the basic kit, $65K in the power train, $30K in the engine, and another $30K in the rest. About 175 grand if you run it in jelcoat and install one race seat.
Maybe you should budget for another 10 or 15K for................the other stuff. We all know how that goes. You will still be a lot under a Porsche.