I've had three speedsters: a CMC, a JPS, and my Intermeccanica. The first car didn't even have door seals or side curtains when I bought it- it was a sunny-day-only car. I loved the car, but wanted more weather protection. I sold the CMC and had a JPS with a removable hardtop built. The car was no where near weather tight, air leaks were abundant- door seals, up the tunnel and out the gear shift linkage, and out the e-brake, at the pan/body junction I mentioned in my last post, in around the wiring penetrations, even the hood hinges leaked air into the cabin. Water cam in around the top bow, the side curtains, etc. By far the worst air leak was at the rear of the side curtains, at the base by the door tops.
I modified the soft-top to greatly reduce the leaks around the side curtains- I believe I had the best sealing side curtain soft-top I've ever seen (do a search on this- I described in detail what I did). I pulled up the carpet, and ran two tubes of caulk into "the crack". But no matter what I did- it leaked, and never made enough heat to drive for 6 months of the year.
At that point, I made a decision. I wanted the kind of weather protection you are asking about. I ordered a new Intermeccanica, complete with a full frame (bonded directly into the fiberglass), rollup side windows, and an Espar gas heater.
It was worth every penny.