IB has a different way of calculating commissions to the other brokers due to its minimum.

Is there any way to backtest the Z-systems taking this into account? It would be helpful for me to be able to calculate the point at which an IB account makes sense.

Yes, but only with your own systems, since you must code complex commission methods in the script. The Z systems use linear commission only.

Except for the upcoming Z8, IB is not suited for a Z system.

Do you mean IB the broker or IB the bridge? Why do you say that? I would've thought that with IB's access to liquidity and low commissions it'd be pretty good, at least with enough volume.

IB the broker. A bridge won't demand margin from you.

IB has good commissions, but bad leverage and lot amounts for Forex and CFDs. Most Z systems are designed for micro lot accounts and leverage in the 1:100 range.