The reason I went with 17s or 18s was to get additional width and better tire choices. You can get 245's in 16" sizes but it's a disappearing tire size. The 17" Fikse wheels are very light for their size but are not cheap. I think 17's are the sweet spot for weight vs. tire choice, and personally I wouldn't go with 18's again.
Weight:
My bug weighs 2050lbs (930kg) with a full tank and a fat driver (the Stig's fat American cousin is 235lbs/106kg) after being stripped and caged. I've got a 45F/55R weight bias, and a 50.5% cross weight with driver. The bug is actually setup pretty damn well and is fairly well balanced.
Tire Temps:
I've struggled with tire temps for a few reasons but first the setup:
Front: -2.5* camber, 0 toe, and as much caster as possible
Rear: -1.5* camber, 0 toe
I expect to have higher inside temps due to the camber and I based my settings on the PCA 70's porsche RS cars. Those cars tip the scales around 2200lbs(998kg) and run 17x10-11" fronts and 17x13" rear full race slicks. I'm running 245/40-17 front and 275/35-17 rear which I think is about as wide as I can go and get good tire temps. That being said I've had a hard time getting the right compound under the car. The 18s I was using had a medium-hard compound which were just too hard really, even in very hot weather. The older race tires on my 17's were a medium compound and good for warm/hot (75F/24C+) weather but wouldn't warm up fast enough for autox duties. I plan on getting a soft compound next which should work for autox and hillclimb sprints, as well as a set of soft-medium compound for track days. I've got a set of Hankook Z214 c71 soft compound tires on order and they should be mounted before the next autox. I haven't decided on a track tire yet and I'm still shopping around.
For tire temps to get into the right range you have to bring the right tire, and I just haven't done that. If you bring too hard a tire out it will feel greasy and just slide around. Street tires can be faster than the wrong race tire.
For the ghia, if you haven't bought your rims yet I'd rethink your sizes. It would be better in the long run to get the same diameter all around either 16's or 17's rather than split sizes. Also do a lot of shopping to see what compounds are available, but I wouldn't run anything harder than a medium compound. The Lotus comes with medium compound yokohama A048's which wear like butter and grip like ducttape (60 tread wear rating), but, those tires don't work below 70F/21C. If it's cold, those tires are icy, I've driven an Exige S 240 track model in the cold and I had more than a few pucker moments. I would suggest a staggered size like 205 front/225 rear but again the same caveat of available compounds, so do some research. Lastly, if it's not an all out track toy, consider the same size all around, easy to find tires, easy to rotate tires (which goes a long way in extending the life of a set of tires), and you can get the same compounds. I go through 4-5 sets of tires a year so if you have a question of particular brands or tires feel free to ask.
Kind of a long answer but I hope I covered it all for you
