Re: power gains
Hi and welcome.
Unfortunately it doesn't mean anything when you saying you have a sports filter and a stainless de-cat.
Some of the exhaust systems done by so called exhaust specialists will losse you anything up to 20% power (althought not often). Many will loose you some power. So can performance or sports filters.
A properly designed system might give you over 10% increase due to the fact that modern Alfa exhaust are rather poorly designed. The same can be said about a prperly designed induction system.
Only way to find out is getting it on the rollers or down the drag strip. But then you need to have the ols figures for comparison.