OP: I went from the IS sedan to the vert, and here's my thoughts.
First of all, like others said #1 is probably due to the sport nature of the car, vs the more plush nature of the solara (which is a 2 door camry).
My question: is the shake/sloshy feeling with the top up or top down. I don't know if you've owned convertibles before, but almost (if not all) convertibles have cowl shake due to design. Some just have it worse than others.
Coming from the sedan, I will say that with the top up the vert is almost identical to the sedan with regard to road feel, if not *slightly* firmer, but that's probably due to having 20000 miles and 4 yr newer car. Also, top up, there is almost no cowl shake at all IMO.
With the top down, there is a little bit of cowl shake. But, compared to other verts I've driven (most recently an 05 mustang gt vert, traded in with sedan for the IS-C), it's *relatively* minimal.
Yes, there is *some* cowl shake, but again that's almost inevitable with convertibles.
For this quote:
"Top up, the cabin is quiet; top down, the body is fairly stiff for an easygoing ramble on good pavement, but even then, we could still feel some steering-column shudder. Softer springs, shocks, and bushings sponge up more bumps than in the sedan, though this weighty ship rolls and porpoises a bit more."
I don't think I agree with the second half of it. To me, the vert seems almost the same as the sedan, with not much porpoising at all. The car seems pretty stiff to me.
Now, with regard to sloshy steering. I can't really help you there. ALL
IS's have a good amount of understeer, which can be relieve with a set of sway bars. But, sloshy steering is not the same as the car pushing. So, if it's not a "push" feeling that you are describing, then it may be something else.
These cars do have an electronic steering rack, and thus, IIRC the "resistance" has to be set. So, it's possible it was set wrong at the factory