Well i meant in handling qualities.
In terms of power and 'hanging' abilities, I realize that the Merlin 66-powered SpitVIII has 1720 hp as opposed to the Mk.IX's Merlin 61 of only 1585 hp.
Also, the speed qualities are not as bad as Karnak stated. The top speed of the LF VIII and XVI is 404 mph (409 approx for the clipped wing 16) at 21,000' while the Merlin 61 allowed for a speed of 408 mph at 25,000' I believe. I know though that the 'LF' Merlins did not impact performance that much. At sea-level, the differences are much more evident. Merlin 66's allowed for 335 mph in real life as opposed to the 315 mph of the Merlin 61. Climb rate was also vastly changed from the 61's max. of 3860 fpm at 12,600' to the 4,700 fpm at 7,000' and 3,860 fpm at 14,000'. At 20,000' the 61 has 2,930 fpm while the 66 has 3,560 fpm. It's not until 24,000' that the 61 has the advantage due to its supercharger 2nd stage kicking in, and only slightly.
But yes, correct me if I'm wrong on my 'tare weight' theory. (Also, I found a document from a book that the Spitfire IX had a rear ferry tank like the Malta Mk.Vc's had... however I can't find any evidence supporting that the IX did carry that small 29 Imp. galloner. I can only find evidence of the rear tanks of 75 IG. If anyone has smoe evidence, I'd like some thanks!)