Lap times from this year Car magazine PCOTY at Cadwell Park:

911 GT3 RS: 53,91s

Caterham Seven R400: 54,28 s

Maybe in a go kart circuit the Caterham'd be faster than the ACR but in a real track the Viper 'd kill it the same way it killed the GT3 RS. In terms of praticality a simple 911 Carrera (or a Cayenne) does a much better job than the GT3 RS, but we are talking about track cars and in this test the lap times speak for themselves.

Who said anything about caterhams? As good asthey are, they don't hold a candle to Radical around a track. The Radical SR8 does the North ring in 6:55. It will destroy ACR or RS around any track.

Well, you talked about Westfields, that are not as good as Caterhams. The Radical probably would be faster, but a car without roof, doors, etc. There are faster vehicles that you can tow to the track than the Radical.

Two posts ago, you said were talking about track cars and lap times speak for themselves, now suddenly roof and doors matter?? Like I said it my first post, yeah these cars are not too practical, but as a track special, they are far faster than an ACR.

Of course it matters. We are comparing street cars and imho the Radical is not one of them. The same way you can tow a Radical to the circuit (absolutely nobody drives them in the streets) you can tow a racing prepped ACR to the circuit.

The R400 was a very fast track car in its day, but it's pretty old now. There is a faster version, the R500 Evolution.

Bedford Autodrome (not a go kart circuit) same-day lap times
Radical SR3 1300: 1:17.10
R500: 1:19.00
CGT: 1:20.20

A GT3 RS will be ~3s slower than the R500.