The answer to calculating the gain of any antenna is usually to model it. EZNEC is a common solution.
Absent modelling, you are correct that a circularly polarized Yagi will have gain approximately equal to an similar linearly polarized one.
The filtering in an SDR is nearly always superior to the filtering in any analog receiver. This is because it's very inexpensive to implement a digital filter which can rapidly approach ideal behavior, whereas good analog filters are quite expensive.
The only analog filtering required by an SDR is that required to avoid aliasing and overload. Since these ...