DansEdgli
The lower the better I say
This is how low mines sitting with chopped springs. I had to remove the bump stops as the car was just sitting on them.
http://members.iinet.net.au/~gileno/cars/xf/xf4.jpg
(If ya want more pics, click on my signature)
I'll tell you now though, chopping the springs does make the ride pretty rough. Im going to have to get shorter shockies as they have about 1cm of movement before they bottom out (which aint good for the shockie towers..etc). Speed bumps are a bitch too.. basically every one touches the exhaust (only about 7cm ground clearance

) .. but thats the sacrafice I've taken for looks
BUT after having said all that, and after seeing DK's photo I'd probably recommend just getting the superlows like he did. It would be a good compromise between looks and handling.. etc. Just get some good shockies too as the stock ones have probably seen better days.
Jaeger
Ive had both 225/65/R15's and 235/40/R17's on my car with its current ride height. Both tyres clear the guard at full lock by less than a centimeter.. so its a very close fit. Under normal driving conditions though (as well as giving the car shit round corners) its never touched the guards.
xa-coupe
I see your point, but i dont fully agree with ya there. Ive been driving around now for at least 2 years with my car this low, and not once have I had any dramas.
I can see how its dangerous (springs popping out..etc), but in my case, even when I fully jack up the front end of the car, the springs barely have enough slack to move around... so i'd imagine going over bumps and such wouldnt be a problem at all.
As for custom springs.. Thats probably the best way to get a real low ride height.. but I'd rather save my money for now.. and spend the $$$ when it gets converted to a drag car
my 2c.

* prepares to be bombarded *