Kas Kastner wrote a book on racing modifications for Triumphs, including Spitfires. In addition, there are some pretty helpful web sites to consider:
http://triumphspitfire.com/
http://teglerizer.com/triumphstuff/index.html
You'll find, as you peruse these and other sites, some talk of "bolt on" modifications. Yes, something will add a little (bigger carb jets, fast road cam, header), but taking full advantage usually involves getting the cylinder head to breath better. The good thing is the MKIII Spitfire motor is normally seen as the best Spitfire motor to work with in getting power increase
I'd leave the machine shop work on a cylinder head to the professionals. I'm currently having to have (expensively!)repaired a rare Mini Cooper S cylinder head (AEG163 casting) because the previous owner attempted to port and polish the head and ground through the lift rod tubes!