You're not doubling the key space. You're increasing it by 2^1024. A 1025 bit key is twice as big as a 1024 bit key. For symmetric encryption, it would increase the time that it takes to brute the key by a factor of 2^1024, or around 10^308, not accounting for Moore's Law. We had a discussion about key strengths a few months ago, and what I learned is that asymmetric encryption, which includes RSA and other schemes used by public key crypto protocols (including PGP), differs in that the key space is the number of primes within some range of numbers, which of course is much smaller than the total range of numbers. That's why 128 bit AES symmetric keys are stronger than 1024 bit RSA asymmetric keys, and won't be cracked until long after 1024 bit RSA keys are cracked. So for Tor's public key authentication mechanism, it won't increase the brute forcing time by 10^308, but it will future proof the keys for many decades. It will increase CPU usage on the relays, and that seems to be the bottleneck on the network right now (not bandwidth). I'm not sure if that increase will result in a noticeable slow down of the network.