i have never used the Sennheiser-E865: (
Mostly a dynamic mic freak here, but I bet someone will know a bit about these.

Why not 3 more 865's ? I realize that the "hypers" are more directional than the "supers" but the differences are fairly subtle. If you are having feedback issues, there are probably other factors in play that are contributing to this.
Where do you place your stage (floor) monitors in relation to the mics?
Also, something that Pan60 alluded to, condensers by their very nature tend to be less "feedback friendly" than most dynamics designed for live vocal use.

I agree with The Moon that you should consider some more E865s. They have the best directional pattern of the 835/845/865 series for this sort of work, but you have to be careful at high frequencies (above about 12 KHz), where the rear rejection is only about 10dB. If you roll off the foldback monitor channels sharply at around 10KHz, you will have more microphone (or monitor) positioning flexibility at the levels you need before squealing sets in.

Assuming your a capella group performs in something like a squashed semicircle, I would use two floor monitors positioned together near the focal point but pointing at right angles to one another, i.e. at 45 degrees to a centre line through the hall. Then arrange your 5x E865s on stands facing up towards each performer's mouth but at roughly 45 - 60 degrees from the monitor line, rather than facing along a line from the monitors. This may take some squared-paper planning and a little care to get the mics' pickup pattern to cover the vocals uniformly and to allow for natural body movement.