Correct, there is currently only 1 TDI engine size/power in the US. There is only the TDI trim and cup edition trim.Here's my experience based on my UK 1.6TDI..
UK SE spec cars came with 16" Atlanta alloys as standard, irrespective of engine type or size.
UK S spec cars came with 15" steel wheels , but S spec was only available on 90 & 105bhp 1.6TDIs, 1 1.6 105bhp NA petrol and 122bhp 1.4TSI petrol (now also 1.2 TSI 105bhp).
The official winter programme from VW shows 15" alloy and 16" alloy options, but the small print states 15" option not available to cars originally factory fitted with 16" wheels.
I asked my local VW dealer, who declined to be drawn on what would fit.
I looked though online parts places to check differences in braking components between models. I couldn't for the life of me see VW fitting different brakes to a SE to an identical engined S.
I came to the conlcusion that 1.6 NA petrol, 1.4 TSI 122, 1.6TDI 90, 1.6TDI 105 all use the same brakes so 15" steel wheels will fit. Another UK owner with a 2.0TDI 140 had also used 15" steel wheels (which confirmed the online parts places data which said my car and the 2.0TDI used the same brakes).
I took the plunge and bought a set of used 15" steel wheels n trims from a mk5 owner on that popular auction site. Â£10 or $15 thank you sir.
Test fitted one to the right front wheel and no issues at all - the clearance between the wheel and the brake caliper is quite small but there is no rubbing or any other clearance issue. So have had Nokian W+ tyres fitted (195/65/15 91T), and will probably mount the winter wheels this weekend and put the Seattles away in their storage bags.
I think the VW warning in their winter programme booklet is really about 1.4TSI 160 models which must have bigger brakes (and presumably 1.8TSI 160 models too).
US versions may be different but if there is a base model with 15" alloy or steel wheels and the same engine as yours you should be able to fit 15s.