Mixed makes of tyres will affect the handling. Of that there is no doubt.
Tyres near their end of life are a lot more unpredictable than those with
even 1mm more tread.
Worn shockabsorbers will also affect the handling in the wierdest of ways.
As a guide, the Poilice always change their tyres when they reach 4mm ! !
Yes, that's nearly double the tread gauge block height. but they do so for
a very good reason...because they want predictable handling and grip,
and they have found over the years that once they get to under 4mm
the ride and handling depreciate at a rather alarming rate.
Ok, so you may think that that is being extravagant. We're all entitled
to our opinions, but if you are driving a Brabus, then to get down all
that loverly power that you all love and want to use, you have to pay
the price. In fact I'd go so far as to say that anyone driving any other
ForFour should change their tyres when they reach 3mm. Your life
depends on those skinny rubber bands wrapped round those pretty alloys.
Face it, for the job they do, in all weather conditions, and the mileage you
get out of them, and the time they last for, they are relatively cheap.
For a normal ForFour, a set of 4 tyres can cost no more than a service.
And you have a service every year....but not tyres. Once every 3 years perhaps ?
As a note, Brabus owners will now find it increasingly harder to get their hands
on 235/35/17 Falken's and Hankook's as it appears Blackcircles don't list them
anymore ! They do however list 215/40/17 in Falkens, Hankook's, and Toyo T1R's.
(18 inch wheels appear to now be the more 'favourite' choice.)
So you may find that you will have to 'change down' from 4 large 8" wide's all
round to 4 slightly skinnier 7.5" wheels shod with 205 and/or 215 profile tyres.
They're cheaper that the 'old' 235's, so I would suggest that what you save with
them you invest in a Bilstein coilover kit (like JJ's) and make it handle better to
start with.