Hi Grant, I never used one of their 'Ring loaners, but booked some trackdays organised by RSR. Everything was very well organised and they have some very experienced instructors.
Some of the old Alfas look a bit "used"
but from what I've heard they are fun to drive. If I recall correctly they bought a new fleet of Exiges recently.
Here are some alternatives (might be worth to compare the various offerings - don't forget the fineprint, especially regarding insurance....
)
http://www.rent-racecar.de/gb/
http://www.rent4ring.de/index.php?id=8&L=1
http://www.hausmarvin.de/en/rent2drive/preise.php
Let me know whether you're more interested in 'Ring tuition or a 'Ring loaner or both - there are more offerings (IMHO the most interesting packages "embedded" in track days) 
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public roads: Porsche 987 S Seal/Cocoa, toll road
: Porsche 997 GT3 Arctic/Black
