Has anyone fitted anything like this before.. am thinking for my Sdi..

Pros and cons???

do you mean a tuning box or a flea-bay resistor special?

In theory, you ought to be able to get a good 15-20% extra power through overfuelling alone - so around 10bhp, potentially, on an SDi. Whether this is a) actually achievable with anything on the market, and worth the hike in insurance, I can't say...

There is lots of information on tuning available using the Search, but I admit it's almost all about forced-induction engines