Yes Coert, on period pictures you frequently see M15 Hessian helmets with regular Prussian spikes, even sometimes on round (not cruciform) bases. Therefore it is legit. On the helmet for sale on the forum there is a perfect color match between the spike and the other metal fittings.
This being said, for me the attraction of the Hessian helmet is the emblem and fluted spike, therefore I would prefer to wait for a 100% Hessian configuration. But this is only my opinion and, once again, the one for sale here by Timp be is very nice and genuine.