To get 20% of the network you have to add 25% to the existing network, while getting 15% requires adding 17.6%. If we assume only 5% expansion per 2016 blocks from everyone else (which is way below what we're seeing) then by the end of September (mid-way through H2 2014) that puts the rest of the network at close to 28% higher than it is now (5 difficulty changes - one's due pretty soon). That puts everyone else at somewhere a little higher than 180 PH/s. The numbers are approximately 32 (15%) to 45 (20%) PH/s.

The 5% expansion rate seems unreasonably optimistic though. If we assume it's 8% then the baseline network goes up to by almost 47%. That would put the baseline at closer to 206 PH/s. Now to hit 15% of the network we'd need to add just over 36 PH/s, while 20% would now require 52 PH/s.