I know the 159 Ti has lowered suspension (15mm
) over the standard 159 and it uses Eibach springs but is it more for cosmetic purposes than for any ride/handling purposes.
Putting 19'' wheels/tyres (235/40/19) will make the rie of the car harsher but will it make a complete hash of the handling as well
it is always a compromise, and what is 'acceptable' for you, may not be acceptable to someone else.
i think 40-series profile tyres is the ideal compromise between handling, ride comfort and looks. the actual tyres confer a lot of this. typically, 19" low profile tyres will be very high quality; this makes for improved grip, less side-side squirm (= better handling) but also stiffer ride due to reduced compliance of the sidewalls. yet different brands/model tyres will also 'feel' different on the road. some tyres (eg: bridgedstone RE050) have very stiff sidewalls, so these feel even 'harder' and the ride will be more 'crashy' and hard. but other tyres (eg: toyo or pirelli) will feel a bit softer and retain very good ride comfort.
in the long term, some other potentially important considerations include:
*replacement cost of tyres: they'll be considerably more expensive; higher quality tyres can also mean softer compound and also encourage you to drive with more enthusiasm (to exploit the extra grip) which means they'll wear faster
*risk of buckling your rims: this is a major risk, and potentially expensive; often not a matter of 'if' but a matter of 'when'; take GREAT car to watch the road extremely carefully and avoid potholes and ridges wherever SAFELY possible; don't let your mates or loved-ones drive the car without this warning either!
personally, i say go for it. the wheels are really nice, and will make a good longterm investment. also consider upgrading the springs and dampers at some stage, not only for looks, but these can also greatly improve ride comfort and handling and overcome the negative effect of the lower profile tyres.