It looks like you've reached the maximum unless you can push the ISO higher by another stop but I don't think the ISO goes higher than 1600 in the 450.
Theoretically, if you can switch it up to 3200 you could halve your shutter speed to 1/500 but you wouldn't change how dark or light they are, it would just allow you a faster shutter speed.
The only thing that doesn't make sense is you couldn't have got an aperture of f/3.5 with the 55-250 if the maximum aperture range is 4-5.6!!
If you wanted a long faster lens Flake is probably the best person to advise on Canon products (I don't shoot Canon) but they aren't cheap. To get a stop faster you're looking at an f/4 at the long end or an f/2.8 lens and they aren't cheap!!
Obvious options would be:
Canon EF 70-200 f/4 L IS USM ~£900
Canon EF 70-200 f/2.8 L IS USM ~£1,500
Tamron 70-200 f/2.8 Di LD SP AF ~£650
Simga 70-200 f/2.8 APO EX DG ~£650
Bear in mind the Sigma and the Tamron aren't stabilised versions either and you can see that, sadly, it ain't cheap!!