Prius OEM Tires vs. Rain Grooves in dry weather

I have a new 2009 Prius Touring -- only about 2500 miles on it so far, and I've not yet begun upping tire pressures etc. as I wanted to first establish an MPG baseline and get a firm impression of various handling characteristics.

I have a stiffening plate which I'll put in in the near future, and I'll consider the strut tower brace as well. But first, what about the Prius in stock condition?

The biggest concern I've noticed is quite a lot of correction being needed when going at posted speeds in the range of 55 - 70 MPH on dry roads with rain grooves. It makes driving the Prius steering very "busy" -- while my other car (2001 Sebring Convertible) doesn't have any reaction to the rain grooves at all. (I do understand the effect of the Sebring's relatively wide and cushy tires .. but they do handle very well.)

What are other people's experiences with a prius on dry rain grooves? Is mine typical?

Have any Toyota/dealer corrections been found -- i.e., is it just bad alignment or something like that?

Does increasing tire pressure make performance better or worse on surfaces like dry rain grooves?

Thanks!