It's a very good question. Trouble is, I don't know the answer.
Having done a lot of heavily sideslipped approaches in our old T21 Sedbergh (the spoilers don't increase sink that much and don't prevent the speed increasing if you lower the nose to try to arrive at the desired spot on the ground so sideslipping is often essential) I can only certify that it works!
You usually sideslip when at normal approach speed for the day (c. 40 knots for the T21) and when you kick it straight, the speed is still about 40 knots. But during sideslipping ASI reading is of course highly inaccurate.