Yep, I have heard that about the 20% on time applying also - sadly I am passed that. Although why we effectively have two levels of airworthiness is wierd. I mean either an aircraft is airworthy or not in my opinion! To say "well this aircraft is airworthy, but that one is airworthier" is strange.
The reason I am wondering is that I am pondering this is that I am just starting my IFR schooling and I would like to take lessons in my own machine (a old PA-28). Sadly here in Sweden (same in UK?) you must maintain it with a 145 shop and "Controlled enviroment" and THEN register it to a school (at a fee! of approx 2000£!) to be able to use it.
In the USA apparently you can take lessons in your own machine without the hassle. Seems much more sensible.