Fielder,
As someone who put a fair few miles (just over 60k in less than two years) on a PD engined Vee Dub I can confirm that Topher really has hit the nail on the head - these things require a total shift in driving style to get anywhere near the best out of them, and you cannot jump straight into one of these (especially with no previous turbo experience, diesel or otherwise) and expect to get the best results.
As for the handling, I can't imagine why you were having problems! I will be the first to admit that these things do lean, but there is no shortage of grip - I'm guessing that the Bora is running on 205 profile tyres on 16 inch wheels. Bear in mind that the mk4 has (or had, until the mk 5 came out) the widest track and longest wheel base of any golf, the available mechanical grip is there - its just the softer springs /dampers (fitted to increase comfort) and extra weight that have a negative effect.
I used to drive the a55 off of my car, no two ways about it - not only did it get a daily caning on a mixture of the UK's roads, but it also did several track days. The natural handling characteristic of the Golf/Bora chassis is understeer (as in most fwd cars), and mine was no acception. There were instances, both on road and on track where the car could be coaxed into oversteer, but this was a result of using weight transfer techniques to get the car to do this. Even under hard driving, the traction/stability control would always minimise what exactly was happening, so I can't understand how the back end almost 'span out' on you??
In no way do I want to sound condescending, as I think you're a great bloke, but I think instead of jumping in a car and trying to drive the a55 off it immediately, you should take some time to learn it.
You are very lucky that you have this airfield available to you (the rest of us have to make do with 2am inclandestine meetings with large roundabouts, or the expense of trackdays to be able to safely explore the limits of our cars) so use this time/space to explore what can and can't be done, not just by barrelling into every bend at 150mph then declaring they car as a pile of crap when it doesn't do what you want!!
Try balancing the car, experimenting what happens with different entry speeds, by lifting off mid bend etc - you will see a far more entertaining side to any car by actually doing something other than simply burying the throttle and understeering everywhere. You will become a much better (quicker) driver, and will know how to exploit the very best from any car.
I can't urge you enough to have a go at a track day mate.