Flash won't run any faster on an older OS.
The two main culprits for Tiger running "sluggishly" compared to earlier versions are Dashboard and Spotlight, which suck up memory and processor cycles in the background. Here is how to disable Dashboard, and Here's how to disable Spotlight. (Although telling Spotlight not to index your hard disk is probably good enough.)
The increase in available software you'd get from having the newer OS in my opinion completely negates the resource usage arguments against it. With Dashboard disabled I don't think Tiger is *any* slower than 10.3 anyway. (Basing this on experience with a 400Mhz G4-ed B&W, anyway.)
Apple strongly links Safari to OS X versions. Running old versions of web browsers is a bad idea in and of itself, and running old unpatched versions of Safari is *extra* bad. (Safari regularly gets P0wned faster than IE at hacker conventions.)