How much "extra speed" then ?
Qualitative, rather than quantitative. I'm certainly not slipping to fly slowly, so if the slip is achieving what I want it to, that's good enough for me.
checked at safe height various non straght ahead slow flight ASI readings
The safe height for this is desk height, before flight, as it's a review of the flight manual position error correction table, for the intended speeds - but knowing that if the airplane has any aerodynamic modifications, the position error correction table in the original flight manual is probably no longer correct. Once you're airborne, there is no way to know the difference IAS to CAS unless the aircraft is instrumented for it.
Yes, I have tested a number of aircraft with the swiveling pitot static head (photo posted earlier in this thread). That is one of the few ways to gather position error data for an aircraft. On my 150, I was able to measure IAS to CAS errors as much as 19 MPH in high AoA and slips. I've used it on a number of modified aircraft. It's particularly useful on helicopters during autorotation, where there is no other means to determine position error. There's also a GPS method, usable in precise level flight, but hardly useful in high AoA, or power idle flight.