hi i am toying with making a decat pipe for my 1.3 spi felicia as apparently it will release 10% more power? will it fail emissions?

the answers are no and yes

ie no it wont make 10% more power, and yes it will almost definitely fail the emissions test

I guess it would also invalidate your insurance.

oh right, prob not worth it then. the other thing was if i replaced the centre box with a straight pipe will the car sound much different?

No, the valve timing on them means there's not a lot of noise anyway - I took the rear box off my recce car, sounded next to no different, so running without a middle box but with the tailpipe would be much the same...

when i took the rear box off mine it sounded properly mean, like a big v8..

my back box fell off one of my old cars once and i drove it around for a week without a back box, i kept revving the engine at the traffic lights and stuff so that i could enjoy the noise, found that the fuel consumption went down from about 40mpg to 25mpg purely cos i kept revving it, so i wouldn't go there again.