They'd be expensive for sure but no doubt some forged Utraleggeras are available at 18" with lower profile tires that would result in equivalent wheel/tire weights. Overall height for bump stop purposes would be nearly identical if you went with a 30 profile. Mind you the ride might be a little harsher with less sidewall... I would probably just do the rears with the larger size - not only would it look better staggered but you would not upset the handling tuning as much other than to introduce a bit more oversteer bias with the stiffer rear sidewall.I would not suggest adding larger diameter wheels on the ST. I have noticed the Fiesta is highly sensitive to weight changes with wheels. Any extra rotational mass needs to be accelerated and stopped. I also think it will be hard to use 18" wheels on the street with the ST. I run 17" wheels with 215/40 17 tires and that is on the edge of what will work for a diameter. Things get a little close on full bump with that diameter.