This all assumes that the Walther barrels have been properly stress-relieved through heat or cryo-treatment after having been profiled to G36E dimensions by a contracted company here in Canada. I have had the unfortunate experience of owning several after-market HK Barrels which were utter garbage in the accuracy department - an HK 416 clone Barrel Assembly and an HK G36K Forend. Both were sourced from (you guessed it), HKParts.net.
So, my experience suggests that mediocre aftermarket HK Barrels are a very real possibility. I am not saying that this is the case here, but caution would be prudent. I'd want to accuracy-test the new aftermarket barrel in my very expensive new rifle without delay. I was able to squeeze honest 2 MOA, 5-round groups at 100m out of my G36E with it's ferociously heavy trigger, back when I owned one. That was elbows on a bench, using 62gr greentip surplus. I'd say that is a reasonable accuracy baseline for any G36 fired under similar circumstances. I'd settle for 2" and hope for better as a trade-off for the shorter service life of the Walther Barrel.