I did it mainly out of paranoia, a few years ago my Impreza track car was breathing a little on the heavy side - but it’s hard to say how much because it’s recirculated and burned off - the only evidence is residue on the inside of intercooler pipes etc. I was consuming oil on trackdays and couldn’t find a leak, so fitted a catch can and realised I was filling a 1litre can every 20 laps The car was moved on shortly afterwards…
My Exige had similar oil residue on the internals of the IC/Pipework so I fitted catchcans to allow me to monitor the quantities and also to keep the IC/sensors cleaner and hence more efficient. I’m happy to report that I’m barely producing 50ml per trackday and almost nothing on the road - but at least I can track it now and my IC internals are now clean as a whistle.
The only downside is the added maintenance overhead of draining the tank, and cost. Operationally there is no downside to your engine, the science suggests that the oil residue/vapour that’s normally in your intake is marginally lowering the octane rating of your fuel (in effect), so you’re also adding a safety margin. ProAlloy would probably suggest you’ll see a power increase too, but I think that’s going to be on the slim side of noticeable
Weren’t Ferrari burning this oil vapor the other year to improve performance of their engine in the F1 cars? Wasn’t it band as a performance improvement?
Yup, Mercedes too IIRC. They don’t put in 5w40 from Halfords though, I don’t know the details but they were boosting the octane rating of their oil to allow for this. In fact I think Ferrari actually had an auxiliary oil tank that contained a totally different grade/composition to that which was used to lubricate the engine, so they weren’t even being subtle about it
I think the teams now have a limit (something like 0.5 litres per 100 miles) which is getting reduced further to something like 0.2 litres in 2020. There may have also been regulatory changes with the oil composition too to stop them adding octane boosters.