Hello, my first post here.
I am not as skilled as Oschkar or other power users here.
I will be interested by your tests, I did some in the past.
All V5500 Pci, not only mac versions, have hardware capability to run @66mhz if receiving from the mobo a "run @66mhz" signal.
Briefly, this signal can be send to the V5500 pci by 2 ways :
-Pci-X slot mobo direct plugging,
-Pci2PciE adapters (adapter must be compatible with Pci 66mhz cards) because the mobo will recognize the V5500 66mhz hardware capability through the PciE lane at the init and send the 66mhz signal to the V5500.
In my opinion, the difference between 33 and 66 mhz (everything else equal in your test) will be an average of 10% more in synthetics benchmarks like 3dmark.
In game, it depends on the CPU dependance, you will score more higher on games demanding a high CPU load.
V5500 will score higher on geometrics tests and the same on fillrate tests. So the fillrate will be the next bottleneck. Overclocking the 3dfx will help at this point, V5500 Pci takes often 185-190 mhz without modification.
At a certain point, with modern Cpu, the benefits are given by the cpu, not by the 3dfx. Some says it is artificial, that's true but if your game runs faster FPS, you will nevertheless enjoy it.
My tribute to the "66mhz way" on Pci-express.
Pictures of my V5500 (2900) dual-slot running @66 mhz.