I've had the Sniper with a dual plane for almost a year now and have no problems because of the manifold.
The thing with Holley though is when you talk to tech support they are inconsistent. One tells you use the spacer and the next time you call they say not too. Before I installed my unit i called and asked about the spacer and i was told, not need, don't use it. Based on that I never tired it and its been fine. I had a few other issues and had the same kind of problems with support, different answer every time you call.