SPEAKING OF THE DEVIL!!! I just did a comparison between the two. I used the same board, a P3V4X.
I have a 600E up to 144 FSB on an Iwill Slocket II (864 mhz), and an 850 Slot I which I tested at default 100 FSB. So both CPUs are running about the same mhz, and you figure the Slocket II setup will win with the 144 FSB, right?
Well yes, it gets better 3DMark scores, but I found in actual seat-of-the-pants gaming, the Slot I runs a bit smoother. Now first of all, I am running all my games at the ragged edge: 1600x1200x32 on a Radeon 8500, all settings absolutely maxed. On games like IL-2 Sturmovik, I get a 'chugging' effect on the Slocket, but it is non-existent on the Slot 1.
Also, take the Radeon Island demos. If you turn 16-tap anisotrophy on the card, the pans get really ragged on the Slocket, but it is discernably smoother on the Slot 1.
There's no doubt in my mind the adapter affects gaming, since the signals have to pass through that big socket 370 on the Slocket, and then to the Secc II socket.
I also tested other Slot I CPUS and they really rip. Now the next thing to do is put that FC-PGA on a true Socket 370 MB like a CUSL, and I PRESUME, it will then perform on a par with the Slot 1.
I also found that the Alpha P3125 Slot I heatsink, though it's a real bear to use since it's so bulky, runs a bit cooler than the Alpha 6035 on my flip chip. I get around 37-38 C stress on that, and around 34-35 C on the 850 at 110 FSB (highest it will go). Voltages are about the same: 1.80. That P3125 is really overbuilt, with the two fans and all.
Lemme tell you, I am impressed with that old Slot 1. It really rips. I am getting smooth gameplay at 1600x1200x32 in NOLF, RTCW, and most the time in Serious Sam Second Encounter! Run an 866EB or 933EB Slot 1 (if you can still find one), something that will guarantee 133 FSB (notice my dud 850E), and it will still serve you well.