I have my GT (that I bought in US) in Europe too. I also drive for many hours at a time at around 1OO MPH. I doubt it will be a problem on the engine, as its only turning over at about 3200 RPM. However I would warn everyone, that although the car goes much faster, it's not a european car. At 120 mph, the car is not glued to the road....and if you lose control, i'l bet you won't recover...where as even in a golf, you might.
In the rain, driving over 60MPH can be a challenge...I've totally had the car sideways in just a slight drizzle. I've been thinking about changing the tires, but I'm not sure there's anything better out there, that's equally good o a Sunny day..????
To improve the Mustang behavior at his speed, you could indeed had some work done on the suspension (lowering the car, firmer shock absorbers, I believe. Though I am not an expert, someone else could give you better infos) and change the stock Pirelli's for some other more performance oriented type of tire. Go to www.tirerack.com
and you will find more solutions.
I am European as well (Swiss), I owned several high performance cars over there and from my own experience I can tell you that rear wheel drive cars with lots of power (say from 270 hp and up, I would say) can
give you problems in adverse weather conditions. Even in a slight drizzle.
I remember my M3, not the latest generation (which I owned after I crashed this one I am talking about), but the previous one (made until 2001, if I remember well...), so the 3.2 liter V6 with 321 hp was a total suicide machine with very, very little water on the tarmac. It would constantly aquaplaning at speeds around 50-60 mph (this I believe was courtesy of the really fat tires it had...very good in dry conditions, a disaster when wet) and you could get sideways even at 10 mph in tight hairpins if you would press on the gas pedal a little bit more than grandpa style when the road was just slightly wet. Even when I got the newer M3 (which has traction control, while the previous model didn't) I would really take it easy on those conditions.
Bottomline is that rwd + lot of hp + fat tires = take it really easy in the rain.
I understand a Golf could manage better: less power, tires that are less wide than a Mustang GT tire (= less water that needs to be expelled from the tire's threads, also more weight from the car itself in less surface gives more chances for the tires to adhere better to the road), less hp to transmit onto the tarmac.
Also about the fact that probably it's easier to regain control of a Golf rather than a Mustang, it's possible: the Golf is much smaller and weighs less, so it's generally more nimble, making it easier to regain control in a bad situation.
This said, I believe you should have taken this into consideration before getting your Mustang ! The package you get, in my view, it's still a gazillion times better than a Golf...