You'll be fine with 6" wide rims.
At tirerack, there's a lower percentage of satisfied customers for the Sumi's compared to the Yoko's. Much less expensive tho.
If you want a pure performance tire, consider Falken Azena. Not a lot of profiles available - there's one in 14" and another one or two in 15". Excellent results reported by autocross users.

With regard to the speedo error, it's pretty straightforward math. Speedo error will be directly proportional to difference in tire overall (tread) diameter. Stock Volvo 165/15 (sometimes listed as 165/0-15 was 25.4" O.D.. If your new tire is say 23.5" O.D. your speedo will read high by 100x(25.4-23.5)/25.4 = 7.5% . Multiply speedo reading by 23.5/25.4 = 0.925 to correct.

Installing smaller O.D. tires provides exactly the same results in performance and economy as installing a rear end with a higher number ratio. You'll get better off the line performance but rpms at cruising will be higher with likely penalty to fuel economy.

Use same logic to select new rear end ratio to correct speedo reading. Me, I'd find speedo reading the wrong reason to change a rear end. Run the calcs at all your local speed limits and post 'em on your dash. That way you won't have to do tricky mental multiplications when the local constable suddenly materializes in rear view window.

Later,
Al