Hi, its very unlikely that you will permanetly damage the input with -40dBm, however with that level of input the preamp is probably in compression, therefore producing intermod and not performing as well as it could.

The 60GHz hear is a bit more expensive, then again it will do Gig throughput. Unfortunately the cheapest 60GHz kit is around $350 for a pair of mikrotiks wireless wire's but 250m is pushing it, hence the cubes, around $400 for a pair is the go.

Cyril