Nice. That's exactly how Thoroughbreds B do.
When I used to have PC2100 RAM, I did some testing and I think we concluded that regardless of benchmarks, every day use was best at 100% in sync, instead of out of sync like 5:4 that you say you're using.
So I decided to avoid the bottlenecks, switched to 100% in sunc and although my PC2100 could only do low 150s FSB, I increased the multiplier to achieve a similar overclock to yours.
In other words, personally, I would do
[150] x 15
100% insync.
Enjoy your system.