1. the gains are reliant on the fair and open behaviour of USA & Japan, and
2. the government might get sued by an investor 

With regard to the first point TPPA at worst makes things no worse;

With regard to the second point there appears to have been plenty of thought and analysis go into the matter.  The people who have invested their lives in getting us a good deal (and have no doubt considered scenarios concerns and possibilities I haven't even imagined) have indicated that the risk is not likely to actually cause a major cost to the country.  In trying to understand the issue I have used a proposed test case of what say the NZ Government decided to ban margarine can Kraft sue? My understanding of what MFAT have explained is that they would not be successful if they did.  So on balance its a risk worth taking.

The remaining question is will the $2.7bn of annual additional GDP arise from 2030 given that 70% of the benefits come from removal of non-tariff barriers.  Nobody will ever know but if you think increased trade is best for NZ then the TPPA is more likely than not to cause an increase in trade.  So regardless of the exact number, a deal that improves trade with USA, Japan and the fast growing Asian economies is something NZ should be part of.

All of that to say I have moved from an opinion of reluctantly opposed to TPPA to supportive.