Historically the Sappenpanzer was reserved mostly for sentries and machine gunners, due to its inhibition of movement. It seems odd, therefore, to give it to the Landser. Going by the name, however, it would be most appropriate for the Pioniere.
As to the MG08/15, it was one of the worlds first man-portable heavy machine guns, and barely modified from the standard MG08 which made it a real bitch to use. Therefore, i believe it should be much less manoeuvrable but also be a more capable weapon, putting it on more equal terms with the Vickers.
German Imperial Army and the Imperial & Royal Army of Austria-Hungary