Job Complete. The L rear shock that was leaking was actually blown out, definitely glad I replaced it. The Koni yellows are stiffer than stock, but not much, and aren't too stiff at all. The racing beat springs have OEM +20% rates, which is not super stiff, very streetable. Overall the car handles much better: less body roll, less squat & dive, more planted, more precise steering. But it is still very street-able, not as stiff as the cars I used to race in SCCA. The car is a little lower but not much, about 3/4" in front, 1/2" rear. The work doesn't seem to have affected the alignment, it tracks straight hands-off even when braking & accelerating, feels right. But I'll get the alignment checked soon just to ensure it's not off a bit causing uneven tire wear.
Advice: Koni Yellows are the firmest, but they aren't as firm as I thought. I was expecting "race firm" but it is really only "sporty firm". I definitely would not want anything softer, like the oranges, TRD or other models.
This is all just about exactly what I wanted to achieve. Replace the bad shock and make it more fun to drive at the same time.
No special tools needed, other than the spring compressors for the front shocks.
Before, back & front:
After, back & front:
Here are the old rear shocks, you can see one is blown, not fully extending on its own (and covered in a thin layer of its own internal oil):