The SWCG cell's output is not related to flow, unless the flow is so low that the salt level between the plates is depleted. But the IC-40 has a flow switch that will shut the unit down before that happens.

VS pumps are slightly more efficient than a properly selected (ie, small as will work properly) 2 speed pump -- but they are MUCH more expensive, complex and considerably less reliable in areas subject to voltage spikes (read: thunderstorms!).

Educated engineering decisions are *always* a trade-off -- there are no perfect choices, or perfect pools, much less perfect engineers.. My experience has mostly been with very large commercial pools, usually designed by a practicing engineer -- and there hasn't yet been a pool lacking a dozen 'designed-in' problems. Even the two pools I designed have some flaws!

Beating yourself up trying make that ideal choice won't help you enjoy the pool: whatever choice you make, I already know it won't be perfect! However, making a "good choice" is often a achievable goal.