I've not used the Marathon Racers in a 16" flavour, but my experience of them at 20" is a resounding 'meh'. AIUI their claim to raciness is that they're lighter weight, not any improvement in rolling resistance, and I'd expect that benefit to become less significant at smaller wheel sizes.
It would appear that they're sacrificing valuable puncture resistance for little benefit.
Happy with Marathon Greenguards on my Brompton. They seem a little faster rolling than the Brompton 'Green', which I didn't expect, and slightly better grip on mud, which I did.