It's not easy researching old sports car stats, so I'll make an attempt to answer my own question with some original research and documentation.
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1966 saw the birth of an incredibly exciting new series, the Can-Am for unlimited sports cars. It also saw the rebirth of Formula One, as displacement was doubled from an anemic 1.5 liters to a more robust 3.0 liters. I followed both, and had attended several of the West-coast big-money pro races for sports cars that preceded the Can-Am. This question delves into the comparative performance of the two cars. Which was quickest? From 1966-74, the life of the Can-Am, 18 races were held at St. Jovite, Mosport and Watkins Glen, while 17 races were held at the same circuits in the same time span by the F1 circus. So some comparisons are possible, especially considering they had many of the same drivers, and tires supplied mainly by Goodyear.
In the beginning, it was clear the Can-Am cars held the upper hand. For instance, by 1967 the Can-Am had been coming to Mosport for 2 years, and Denny Hulme (McLaren M6A-Chevy) had set pole at 1:20.8 and fastest lap at 1:20.7 (109.7 mph). The '67 F1 pole by Jim Clark (Lotus 49-Ford) was 1:22.4, with FL at 1:23.1 (106.5 mph).
However, by the end in 1974, it was clear Can-Am, over-regulated and lacking competition, was slower than F1. Indeed, in a demonstration match at Laguna Seca between Don Nichols' Shadow Can-Am cars (Oliver and Follmer) and his Shadow F1 cars (Jarier and Hunt), the heavier iron was humbled, spun and lapped.
Formula 1 vs Can-Am sports car - which was faster?
1966 Can-Am
St. Jovite.......Pole 1:38.4....Fastest Lap 1:37.3 (98.05 mph)
Mosport.........Pole 1:50.7....Fastest Lap 1:23.1 (106.5 mph)
1966 F1
Watkins Glen...Pole 1:08.42...Fastest Lap 1:09.67 (118.846 mph)
1967 Can-Am
Mosport..........Pole 1:20.8....Fastest Lap 1:20.7 (109.696 mph)
1967 F1
Mosport..........Pole 1:22.4.....Fastest Lap 1:23.1 (106.5 mph)
Watkins Glen...Pole 1:05.48...Fastest Lap 106.0 (125.455 mph)
1968 Can-Am
- no races these circuits -
1968 F1
St. Jovite..........Pole 1:33.8....Fastest Lap 1:35.1 (100.315 mph)
Watkins Glen.....Pole 1:04.2....Fastest Lap 1:05.22 (126.955 mph)
1969 Can-Am
Mosport............Pole 1:18.2....Fastest Lap 1:19.5 (111.25 mph)
St. Jovite..........Pole 1:31.7....Fastest Lap 1:33.8 (101.7 mph)
Watkins Glen.....Pole 1:02.21...Fastest lap 1:02.6 (132.27 mph)
1969 F1
Mosport............Pole 1:17.4.....Fastest Lap 1:18.1 (113.345 mph)
Watkins Glen.....Pole 1:03.62....Fastest lap 1:04.34 (128.691 mph)
1970 Can-Am
Mosport............Pole 1:16.8.....Fastest Lap 1:18.0 (113.492 mph)
St. Jovite..........Pole 1:33.0.....Fastest Lap 1:34.3 (101.17 mph)
Watkins Glen.....Pole 1:02.76...Fastest Lap 1:05.8 (125.84 mph)
1970 F1
St. Jovite...........Pole 1.31.5.....Fastest Lap 1:32.2 (103.467 mph)
Watkins Glen......Pole 1:03.07...Fastest Lap 1:02.74 (131.973 mph)
1971 Can-Am
Mosport...............Pole 1:17.3.....Fastest Lap 1:18.8 (112.343 mph)
St. Jovite.............Pole 1:32.9.....Fastest Lap 1:33.6 (101.93 mph)
Watkins Glen........Pole 1:05.11....Fastest Lap 1:06.083 (132.84 mph)
1971 F1
Mosport................Pole 1:15.3.......Fastest Lap 1:43.5 (85.522 mph) (rain)
Watkins 5.434km...Pole 1:42.642...Fastest Lap 1:43.474 (117.495 mph)
1972 Can-Am
Mosport................Pole 1:14.2......Fastest Lap1:15.2 (117.766 mph)
Watkins Glen.........Pole 1:39.187...Fastest Lap 1:44.300 (116.560 mph)
1972 F1
Mosport................Pole 1:13.6......Fastest Lap 1:15.7 (116.929 mph)
Watkins Glen.........Pole 1:40.481...Fastest Lap 1:41.644 (119.611 mph)
1973 Can-Am
Mosport................Pole 1:14.1......Fastest Lap 1:18.0 (113.492 mph)
Watkins Glen.........Pole 1:38.848...Fastest Lap 1:40.0 (122.74 mph)
1973 F1
Mosport................Pole 1:13.697....Fastest Lap 1:15.496 (117.256 mph)
Watkins Glen.........Pole 1:39.657...Fastest Lap 1:41.652 (119.596 mph)
1974 Can-Am
Mosport................Pole 1:14.5.......Fastest Lap 1:14.6 (118.66 mph)
Watkins Glen.........Pole 1:39.969...Fastest Lap...........(119.35 mph)
1974 F1
Mosport................Pole 1:13.188....Fastest Lap 1:13.659 (120.181 mph)
Watkins Glen.........Pole 1:38.978....Fastest Lap 1:40.608 (120.837 mph)
1973 Porsche 917/30 Can-Am Spyder
- 1100 hp race, 1580 hp qualifying
- in a special test, lapped Talladega at 221.160 mph