I think it would be really difficult to make a straight comparison... They only have six years of arithmetic before they start Algebra and Geometry in mid-7th, so to that extent you could say they're two years ahead of a 9th-grade-algebra scope and sequence, but then again the 9th grade timing isn't as standard as it might have been in the past...
I think in 1st grade the difference is minimal, but it grows along the way. The tests are very true to the Singaporean scope and sequence (so they're reliable for placement) but if you're switching between curricula many people find that their kids place way WAY back just because of the different approach and order of topics, and then they catch up fast. So based on that I'd say it's just hard to compare.
If you needed a US curriculum placement, you could try to find a Saxon placement test... it might be closer.