I thought Ian might have pitched in. I remember he had 400x19's at one point. But I think he said it affected the handling to some degree and was a bugger to remove/fit the rear wheel as he had to let the air out to get it past the top of the mudguard. 
He then reverted to 350's. Ron
email (option): ronpier@talk21.com
Yes...I used 400 Dunlop SM tyres...With these fitted I did have to deflate the tyre to remove the wheel as Ron recollects...There was also a clearance issue with the cross bar on the rear stand...
The SMs have a 100% aspect ratio I believe (the height of the sidewall in relation to the tyres width) and that was a factor in that case...
I fitted one on the front as well and though I liked the 'chunky' look of the tyres I thought they made the bike handle like a barge with heavy steering...
At the end of the day I suppose it's down to what you like the look of and what you feel comfortable with on your bike...Tyre performance is a very subjective thing after all...
I reverted to my usual 350 x 19 Dunlop K70s at both ends to eliminate the clearance problems and to restore the more 'nimble' handling...which is a relative term on the M20...
...Ian
email (option): ian@wright52.plus.com
That'll be Avon SMs (safety mileage)..not Dunlop...Ian
email (option): ian@wright52.plus.com