I don't think the bump was fully needed - forums just not that active at the moment.
In terms of the filter though, 150 gallons is 567 litres and 1 FX6 is 3500 litres so that means about 6x turn over which is just about ok. When I had my last 500 litre tank I had 2 FX6s on there so I had a back up, but I'm not 100% we needed the turn over. Worth bearing in mind.
I have mixed views on the FX range, I had mine set up for about 8-9 years and in that time I think I must have had 6 filters altogether as they all broke in various different ways. There is now a Sun Sun knock off FX6 which is just a white version of it, the Aquael externals look very good too as they have a pre filter (and I just like Aquael at the moment).
I think initially you should be fine with just one, its just the reliability that would make me want to have 2 - just as a back up.
Wills