I've never known of any manifold adaptors. I have read that people have welded on the st205 plenum onto the inlet runners of the st185, but not retrofitting the whole inlet manifold.
More commonly, people swap the st205 head onto the st185 engine block (and change to a steel head gasket as well)
From my research over the years, the centre feed inlet manifold for st185 and st205 does not have significant difference.
The throttle body opening is larger, st185 = 55mm, st205=60mm, st215=65mm (from memory, maybe I should measure)
http://gtfour.supras.org.nz/3sgte%20table.htm
1. People either change the throttle body to an aftermarket, or retrofit the st205 throttle body to the st185 plenum. Requires wiring in the throttle position sensor and other sensors as the st185 sensors cannot be fitted to the st205 throttle body.
2. People change the entire plenum to aftermarket with aftermarket throttle body
3. People change the head to ST205 and have the st205 plenum and other ancillaries as well
More information:
viewtopic.php?f=7&t=3868&start=0
To optimise your current system, I would not look at the throttle body as a significant weak point. CT20B will run up to 17psi / ~1.2 bar boost safely so you will only be looking at approximately 300BHP engine / 170kw at the wheels (factor in 23% loss from engine to wheels). Might be a bit more, but the output is about 170 at wheels with CT20B and stock 440CC injectors.
As this is a turbo application, you are already forcing air into the plenum and 5mm isn't going to make significant difference with the CT20B.
Hope this helps.