Is anyone familiar with the 100 lb gross weight increase for the C-172N model available from Powerflow Systems?  I was on their web site researching the exhaust system upgrade, and saw that they also have a gross weight increse STC, no exhaust system upgrade required.  Their advertized price is $300, but I would be interested in hearing from anyone who has done it.  I would like to know what the real cost is on an unmodified 1978 C-172.  The $3500 for the exhaust system is out of reach right now, but the 100 extra pounds would let me travel with my wife and both my adult children (considering density altitude and runway length, of course).