I think it will be a tad bit more than 20%, and then there's also the fact that it's got 64-bit processing
If you go from 1.83Ghz (T2400, 2mb cache) to 1.8Ghz (T5500, 2mb cache, 241$)
You should see 20% jump.
But if you go from 2.0Ghz (T2500, 2mb cache) to 2.0Ghz (T7200, 4mb cache, 311$).. Then you will see a bigger jump.. Twice the cache, at least 40% jump.
And Im buying my laptop in august, so im thinking of buying directly a Merom processor. I'd be the only one in university with a merom processor in the first day of school probabily. A 1500$ laptop will smoke the sh!t out of their 4000$ Sonys or alienwares.
There are problems with this, such as voiding the warranty, and IF the custom BIOS is buggy, you might fry your whole PC.
So unless ASUS or someone reputable releases a BIOS, I'm not 100% sure if Im gunna pop a Merom in yet.