if i remove my centre pipe and cat and replace it with a straight pipe will this give me a deeper louder burble

 

Yes, depends on what it's replaced with, but yes.

 

i have a magnex back box so was going to put on a magnex straight middle pipe

 

Haven't heard the Magnex (someone on here will have one), but it will give you a different sound. Should be louder and deeper. Mines custom fabricated and sounds great, not too loud, but deeper and really brings out the burble.