Since this has been brought up many times and I was directed to a post in the Motorsports forum I thought I would repost my response here
So, let's look into the issue of shorter tires for AutoX ......
A P-Metric tires max load is achieved at 35 psi and a Euro metric tires max load is achieved at 36 psi (XL or RF tires at 42psi). Adding more air pressure will not increase the tires load capacity but, lowering it will reduce the load carrying ablility. When increasing pressure all you are accomplishing is a lower rolling resistance, quicker steering response, stiffer ride and contact patch reduction.
Based on a WRX:
the OE tire is a 205/55-16 89V
max load capacity is 1279 lbs.
load capacity at recommended pressure (which we will call PLC for placard load capacity at 32psi) = 1213 lbs.
so the minimum load capacity for a WRX is 1213 lbs. according to the PLC
now if you look at the optional tire "OK'd" by Subaru
max load is 1201
PLC = 1135
so in reality the minimum load capacity is 1135 lbs.
now lets consider the idea of running a capacity that is too low .... what happens:
the tire is put into a condition where it can support the car at a static rest point but, when driven aggressively the sidewalls are so over worked they can not properly control the contact patch of the tire allowing for excessive sidewall roll-over, irregular wear tread block and contact patch flex which robs the tires performance ability and grip. also the added flexing of the tire will increase the heat it generates which can make a tire feel really slippery .... better gear ratio is achieved but if you can't utilize the power it doesn't matter ....
A good way to understand this is: let's say I just put a 100lb. back pack on you .... you can stand there for quite a while (that's your car sitting in a parking lot) now go run up and down some stairs with that back pack .... that's normal driving and you can't do that very well, nearly as fast or for very long before your legs give out .... after you've recovered enough to walk (let the car rest once you arrive at the venue for your autoX), take a 500 meter sprint through deep sand, that's your autoX and you will most likely not make it to the finish without falling atleast once if you can finish at all.
there are possibilities for a shorter tire though
alternate tire sizes that would work on the WRX while maintaining load capacity (these may or may not be all of the sizes that could work and some of these may not even fit)
215/45-16/1168 **** minimum 36 psi
all of these tires would be acceptable as long as at least 35 psi was run in them except where noted