When running the 100/133 operation, VIA called this mode pseudo-synchronous because while it really isn't that much different than the 133/133 mode, it operates by buffering the incoming data for a short time and denying requests from the memory that may come in faster than the bus needs.
Frankly, you lose a little performance- but I can't say it would be more than 5%
The thin with the Athlons is that they REALLY like their FSB bandwidth to be clear so using the 266 speed does improve performance a little.
Eric