OK, this is purely my take on the whole thing so someone please jump in and explain if I have it all backwards

I doubt 17's are the favoured option for handling on a light car like the Escort.The heavier the car the more relevant side wall flex becomes due to the side forces exerted on the tyre. On a light (comparitively) car like an Escort I think 16's would be the better compromise and 15's being ideal.
On a car prodominantly used on the road the extra side wall can accomodate a firmer spring and shock rate but also retain some compliance. Going from 15's - 17's would mean this firmer setting might become unbearable and also be detrimental to handling. On the track this may also be the difference at the limit by allowing a more compliant suspension setting.
Have a look at all the race guys driving Escorts - the majority run 15's. Also look at all the kit car like Westfields etc - 15's is the preferred option also. What about the side wall heights on the F1 cars - once again it's the lack of excessive weight.
Plus (and more importantly as already mentioned) is the legallities of the whole thing.
There you go but it's only my opinion