Well, clear now, that will be better than the plastic one.
You can find it for example here:
http://vanmoof.com/shop/catalog/product_info.php?cPath=112_126&products_id=214
http://www.strida.tv/pieces-detachees/154-alloy-freewheel-.html
Sorry, found no direct link at http://strida.es/
I thought maybe you’ve meant the aluminium upgrade for the front belt wheel:
Our new chainwheel set for strida 5.0
ChainWheel replacement for Strida 5 ?
Unfortunately - no, just one with less teeth (80).
And btw, YOUR effort (and higher cadence) alone can make you faster, without a larger wheel 
Here I’m again unsure what you mean exactly…
For the developments and gear ratios of the 16" and 18" Strida wheel sizes please refer to here:
Gear ratios information about different models of Strida
If you want to use the decathlon bfold 5 please note also that this frame would allow 20" road wheels - with the 100:30 ratio you would be anyway faster (at the same cadence).
For myself, I’m using 80 teeth front and 30 rear - due to the speed drives.
My usual speed on flat surface, without wind and less effort will be with 16" wheels slightly below and with 18" wheels a bit above 20 km/h.
But I’m not trained, lazy and ride for short distances only - I’m surely a bad example for a benchmark 
You’ve also asked for the prices, please look at your pm (upper left corner) 