Actually you can get 2.5" drives as fast as 10K now, and a lot of manufacturers are moving to the smaller form factor to reach speeds as high as 15K (SAS).
Having a 2.5" over a 3.5" actually has some advantages too; generally less power consumption, less heat generation, slightly faster spin up times compared to 3.5."
Regardless it should work without a problem, just find the mounting brackets and you are good. I know OCZ makes brackets but I don't recall if they are just for SSD, which the ones I'm thinking of may be.,
Our new IBM SAN (at our Oakland location) at work uses 24x 2.5" 10K SAS drives... blazing fast!