First off, as a chronic tinkerer, I always try to upgrade fans in my amp, especially if you can barely feel any air moving. I think you are doing a great service to your amp and its lifespan.
Does your fan have the current draw/power consumption and CFM listed on it? Most fans list this info on a sticker in the center of the fan, or on the outside edge. You want to make sure you don't exceed the current draw or you could melt wires inside your amp and possibly cause a fire (no! lol). Also if you know the CFM then you can assure your new fan will move more air. Lastly, it helps to know if it's a brushless design, as some amps can have noise issues with a fan if it is not brushless. That's because motors with brushes have a small electric arc between the brush and commutator while the fan is running. Wires and components inside the amp can pick this up as EMF and can cause a whining noise through your speakers. If you have a brushless fan now I would definitely get another one that is brushless, otherwise you could have issues. In fact, just get a brushless one regardless, it's cheap insurance.
Don't forget that a higher CFM fan is usually going to make more noise. I believe you can find some that are designed for low noise. Not really a big deal playing live, but if/when you hit the studio you may have to put your amp in a separate room from the miced cab if you have this turbo fan making a bunch of noise.
Let us know how things go.
I'm a weapon of mass distortion.