I strongly recommend everyone supporting this proposal to read the last performance report for the network after the increase of validators to 300 slots.

BABE and GRANDPA are running smoothly with up to 1000 validators, but the parachains consensus logic is far heavier and more complex and will need optimization to reach that point. As such, the parachains team is planning to only use some limited subset of the validators initially (perhaps 50, 100, or 200) randomly selected each session for parachain consensus. Those selected validators to do parachain consensus will receive the bulk of the rewards for each session. Over time, the amount of rewards each validator receives will be roughly equal, but increasing the validator count relative to the amount of validators participating in parachains will lead to increased variance of rewards. For that reason, the parachains team recommends keeping the amount of validators low to achieve an outcome of low reward variance, an earlier launch of parachains and consensus stability.

A drastic increase like the one proposed by the this particular proposal (to 499) can bring some issues to a network focusing at the moment on stability for optimization.