What is the correct/best offset for these wheels? I want to do 20" diameter and probably 9.5" front and 11" rear - unless those are bad widths for some reason.

Input in welcome! I am thinking of DPE MT5's.

Anyone have some insight on this?

I suggest 20" HRE P43's.

I've been working with a couple wheel companies, and I'm going to stick with 9 in the front and move to 10.5 in the back (20") with offsets of 30 & 40, respectively.

I have a nice set of 20" that should fit (see sig for details). If you are in the LA area, you can come over & we can do a test fit so you can see for yourself.

A naive question (anyone may feel free to ):

If I were to swap out the 19s my car came with for 20s, wouldn't the speedometer and the odometer give incorrect readings? If so, can these be re-calibrated?

Yes and No. The no meaning that I have never seen any discussion in non-racing forums about correcting the 1-2% error on the speedo. For the mileage, the error is in your favor with increased diameter.

I believe that the tire's outer diameter & circumference would stay the same - just a thinner sidewall to accommodate the 20" wheel. I think the issue is if you put a larger tire on the car. But I would be wrong...

Surge is right too - I was assuming that you would keep a similar sidewall, as our cars already have a pretty short sidewall. With an overall increase in diameter, the speedo / odo will change.

When you increase from 19" to 20" you will also decrease the profile of the tire (smaller sidewall). The important factor is keeping the rolling diameter the same. Of course when you decide to increase the width of the tire by 10mm, you will typically change the speedo by 0-1%. Also keep in mind the factory speedo typically is not always accurate. Many tests have shown that with a stock vehicle, the speedo is incorrect by +/- 1-3 mph.