It’ difficult to say if additional buoyancy under the seats would make a difference to inversion. Anything that makes the boat sit higher on the water when capsized will encourage inversion. But buoyancy under the seats might be offset by the weight of your cruising gear.
I would emphasise that the important point is for your Wayfarer to pass the buoyancy test – make sure your forward and aft lockers have good seals. And that there are no problems around the grp joins – the deck/hull join is prone to leak as this is often subject to collision damage.
The test allows some water to ingress – it should not be 100% waterproof.