You do generally get what you pay for....
I have to say peoples buying attitudes have changed a lot recently, there are a lot of posts about buying the cheapest this and that.. im looking into cheaper products to offer too.. its not just car parts its life in general.. and its understandable in the current climate
However (speaking as an owner not a trader), if I were you.. i'd sooner spend £120 extra ( a night out in Notts for me ) on an exhaust that you know sounds good, is generally well trusted and has a lifetime guarantee .
You posted," if it doesnt fit I will get my money back." yes you will, however... who pays the garage that tried to fit it and who pays the return costs for sending it back to the supplier ??? well that would be you, and thats probably your £120 difference...
Having said that it might be perfect