Porting, 34mm carb and lightened flywheel will make a huge difference. Look at AAEN stage 4 kit. This will run with 400's. The reed cage is also a big performance gain. You will never beat a Banshee with the 250 so thats just out of the question. If you had the 400 scrammy then yes you can build it to easily beat banshees but not the 250. Here is what will beat the 400's.

Pipe and silencer
ported
34-38mm flatslide carb with reedcage conversion
lightened flywheel
head milled
uni or K&N filter
rollar cluth kit

The ultimate performance would be a stroker kit with reed cage and all the rest mentioned above.

You have a good start on mods. If you wanted a some more power but not break the bank then get it ported and a bigger carb or ported with a reed cage conversion. Keep it simple and relaible. If you start doing other mods to try to beat 400's you lose reliability.