The TTXGP/Zero winning times are:

2009 - 87.434 (25:53.50)
2010 - 96.820 (23:22.89)
2011 - 99.604 (22:43.68)
2012 - 104.056 (21:45.33)
2013 - 109.675 (20:38.461)
2014 - 117.366 (19:17.300)
2015 - 119.279 (18:58.743)

The speeds are now close to the twins (lap record is 15 seconds slower) so from that point of view maybe it is time to increase it to 2 laps. The low entry numbers though will surely decline further if the distance is doubled. After all the 6th place finisher (last in 2014 and 2015) has gone from 81.515 (2013) to 77.956 (2014) then 73.156 this year.

I agree with DBD that maybe a stock 600 would give a little more variety if replacing one of the supersport races. It may give some of the privateers more chance to run with a little less spend.