http://www.firstworldwar.com/battles/guise.htm
Interesting idea. It depends on how quickly it can get there and what condition it is in when it arrives. It would have an important impact at the Marne, where the extra corps may well cause the final breach of the French 9th army, forcing the 5th to divert critical troops back to the marshes of St. Gond. That may well prevent the rapid breech of German lines, allowing them to hold on the Marne river instead of the Aisne, which in turn means the German 5th and 4th armies can continue to bash up their rivals around Verdun, which will probably mean Verdun is cut off and the French 3rd army is badly bashed up. Sarrail is then gone for sure.
Basically the German V corps doesn't give up at Troyon because they aren't ordered to fall back to conform to the retreat to the Aisne. Of course there is plenty of supposition here on my part and things could play out very differently, but its more likely than just a replay of OTL.