The biggest problem (IMHO - but I'm not a "rivet counter") is undercarriage covers (early, "flat" type in the model, but it should be the late type) and other (shorter and thick) cannons. The other thing are decals - it looks that upper wing roundels should be smaller (C1?). Something is wrong with the shape of carburateur intake, but I didn't care about it.
The fit is quite good, Mk XVI was the fifth ICM Spitfire I've built and I didn't have any bigger problems except fitting the engine - if you would like NOT to show the engine, don't use the "firewall" part - it makes the fuselage too wide and thus the dihedral of wings is incorrect. - this is the part on the picture, to which the engine is connected
i4.photobucket.com/.../hip002.jpg
And since I'm just about to finish my Mk XVI - I can show you my model (I'm sorry to interfere in your post, but it's only to show how it looks) - on these pictures my Spitfire hasn't proper cannons yet
i4.photobucket.com/.../IMG_1508_zps377de67c.jpg
i4.photobucket.com/.../IMG_1510_zpsfd8562fd.jpg