How much power do I loose if my tires slip on the dyno rolls?

It can be significant and it varies (which is its own problem)! For repeatability, you must not have tire-slippage percentage entering into the equation. The major sources of rear wheel horsepower losses are due to tire to roll slippage and sidewall deformation. Both waste energy as heat transfers into the tires, rolls, and surrounding air.

Deformation is often worsened as operators strap the vehicle down harder, desperately trying to improve traction. Although the extra contact pressure reduces slip the once per revolution flexing of the tire over the roll eats up a good chunk of what was recovered by lowering the slippage - while inflicting hidden damage to the tire!

The solution is larger rolls with machined-in deep traction grooves. They provide twice the traction (μ > 1.00) compared to smooth or cosmetic diamond-knurled surfaces (μ < 0.60), and require only a fraction of the strap-down forces!