Interesting question, not a 100% sure on the answer.
Here's my best guess......
You would hold a restricted PPL-IR based on a foreign license.
If you take the Commercial Practical test your temporary license will read something like;
" Private pilot Instrument based on foreign license, Commercial priviliges VFR only"
You would have to take an Instrument checkride to get Instrument priviliges on your Commercial.
Therefore it might be easier to take the Instrument checkride before the Commercial.
The IR priviliges on your restricted are meant to be used to fly in the US under IFR, not necessarily as a basis for a future US rating.
Hope it helps.
