Tom,
First, there are differences between the manufacturers. For example, JPS and VS are built on a VW Beetle pan. Beck is built on a pan-replacement tube frame, with swing-arm rear suspension. IM is built on a full tube frame, and has IRS suspension.
Another example, JPS and VS use side curtains. IM uses glass side windows, available as rollups or power. Not sure about Beck.
As you can see, the cars will handle differently depending on what the chassis is. And they will handle wind and rain differently depending on side windows etc.
That said, most if not all these cars will handle fairly well. If your car gives you a bone-jarring ride, your suspension is not set up right. Straight from the builder, the ride should be good.
Seats will also make a difference to your comfort. In addition to a Speedster, IM offers a Roadster, which comes with a slightly taller windshield, and much more comfortable seats. The 'original' speedster seats are not very comfortable, though they do hold you in position well when swinging through the twisties.
As for long trips, so long as the engine is built right, you should have no problems. A smaller engine will have an easier time staying cool.
Noise is a relative thing, and also the source of the noise makes a difference. For some wind noise bugs them, but the roar of their engine is music to their ears. An IM with roll up windows will have less wind noise than a car with side curtains. All these cars have wind noise when the top is down. Engine noise is tougher. The smaller the engine, the less it will roar. Next is a good exhaust. Henry of IM has a new exhaust that is very quiet using two mufflers, but it makes valve adjustments more work. You win some, you loose some I guess.
So, have I helped or just added more mud to the water?