The +4 is nimble and chuckable, the wider tyres will possibly hang on for longer BUT when they go you will be travelling at higher speed and catching them will be that bit more tricky. I think it was Tim (Hamwich) who said you want to choose tyres which you can break traction at low speeds to have fun, and will be controllable when drifting at higher speeds. Wider tyres won't help in either respect.
The 205/60 x 15 is a standard size for the alloy wheeled +8 , and I think that it over-tyred, a +4 needs a narrower contact patch to maintain its handling.
I did indeed say this, I am firmly in the camp that says that on a Morgan one should go for the narrowest, tallest profile tyres one can fit in order to maintain lightness, agility and fun factor, bu tI acknowledge that many may prefer a more sedate 'planted' feel on the road - in which case the critical factors would be clearance and wing width, as others have pointed out.
My 4/4 has 165bhp, weighs around 860kg, and is perfectly happy on 165 section tyres. A 2013 Plus 4 produces less power but has a wider stance and so is probably better on the 195 tyres but I can't see that going to 205 section would be a good idea unless one really really liked the look and was happy to sacrifice a deal of handling performance to get it.
