What weight BBs are you using? If you are using, for example, 0.25g, being just below 300FPS is not a problem. If you are going to work on a gun, you really need to buy a chrono.
What did you lube with silicone oil? Gears should not be lubed with it; you need to use something like Super Lube or gun grease. It is possible that you have got silicone oil on the hop rubber, which could explain the very short range.
A new spring is only worth investing in if doing everything listed does not drag the power up to where it should be. If everything is sealing, you should only need an M100 or M105 spring.