the 5.7 has more or less the same power as 9mm, it is powerful enough to use the lock-breech design and will generate some good penetration + tumbling, even from a 3" barrel.
A single stack mag will probably hold 8rd of 5.7 instead of 6rd of 9mm. If the mag has the same width as the 9mm mag, the rounds can off-set a little so it might even hold 10rd. The 5.7's case is tapered which will enhance reliability a lot.
Is it possible? It seems that the grip area (front to back) will have to be lengthened a bit and so is the mag, barrel/breech face/extractor/ejector will be different, but that shouldn't be too much work for the adaptation...
The only 5.7mm handgun currently available is already around $1k, a R57 with a $1300 price tag will not be that much more $$.