With the information you've provided, there is no way to tell. Feedpoint impedance and VSWR have no relationship to antenna efficiency. See What is the relationship between SWR and receive performance?
The feedpoint impedance also has no relationship to antenna efficiency. A "50Ω antenna" means the impedance at the feedpoint is 50Ω. The impedance at other points is probably something very different. In a dipole, for example, the impedance is about 72Ω at the feedpoint, and very much higher at the ends. This is why current is at a maximum at the feedpoint (low impedance), and at a minimum at the ends (high impedance).
Is there an advantage to an antenna with a feedpoint impedance equal to the impedance of free space? No. An antenna of any design transforms between the impedance of free space to its feedpoint impedance. An antenna can be designed to work at any feedpoint impedance. The losses come from things like resistance in the antenna wire and coils, and dielectric losses in tuning capacitors. To the extent that you don't have these things, an antenna can be efficient. A dipole, vertical, loop, or any other kind of antenna constructed from ideal materials is already 100% efficient.
This can easily be seen by considering the law of conservation of energy. If your antenna isn't getting hot, yet you are putting 100W of power into it, only two things can happen to that energy. It can be be:
- stored in reactance of the antenna
- radiated away
If the antenna has a purely resistive impedance, then there is no reactance to be storing energy, so it must be radiating away. The feedpoint impedance is irrelevant.