In terms of economy, Ive chipped a few 3.0 MV6's in the past, using Superchips, Courtenay, Ex Plod eproms and some from Germany / Poland.
The first two gave improved MPG particularly noticeable on a long cruise, a 3.0 MV6 Manual Estate (Superchip) went from never doing more than 34 to 38 MPG (28 around town) and a 3.0 MV6 Auto Estate (Courtenay) from never doing more than 33 to hitting 40 MPG (25 around town). Cant honestly say I noticed any significant improvement in performance on these two.
The Explod one I got my hands on did make MPG worse but it did come with widened throttle bodies couldn’t get more than 33Mpg on a run and around 19 in town, but did give a noticeable improvement in performance though.
The ones from Germany or Poland didn’t seem to make bugger all difference to MPG or performance.
All the chips Ive used have never thrown the EML on, and would I pay 300 / 400 quid for one, No.
Chris.