IMO its the offset of the caliper to disc with respect to the ball joint that is the main issue.
Having gone through the ST205 caliper option, it just isn't as easy as swapping over due to the superstrut design.
Looking at Kris'/gt4dc photo of the superstrut, the location of carious components means the disc has increased offset, therefore the caliper is also has increased offset compared to macpherson strut.
At the end of the day, you need to decrease the offset of the disc by using spacers and then match the caliper to mount properly based on where you have offset the disc. On MacPherson strut, the disc has to be offset to clear the ball joint.
In the end, I just gave up, got wrx STI discs, got a hubcentric ring made up, got some wilwood superlites with a bracket, got new lines made up and it was done. I get clearance of the ball joint as STI disc is lower offset.
st205 disc + st205 caliper
I don't have to modify a consumable aka the disc, to fit the st205 caliper when mounted on the front of the hub.
I don't have to get a spacer and longer studs to space st205 discs out.
wrx 294mm disc + st205 caliper
No pad spacers due to smaller disc width 24mm vs 32mm
No overhang of st205 pad to small disc cirumference
No spacer for decreased disc offset
Altezza/IS300 disc + st205 caliper
No re-drilling of disc to 5x100 vs factory 5x114.3
No machining of caliper or hub mounting point (need to shave 2mm off either)
Pay for solutions that work up front
Don't mod consumables or compromise integrity of components
If there was a cheap solution out of the toyota parts bin, then fine, but everything that uses the st205 caliper is a compromise in my books, so I just saved some coin, specified what I wanted to engineer and he sent me a kit. Means I don't have 'CELICA' on the side of my calipers, but meh - I have good brakes with good engineering and consumables I can buy off the internet - easy peasy when the hard work is done up front