Earlier quoted context omitted.
That's a reasonable upper bound. Once the company is big enough to the point where you don't remember everyone's name, it's past the startup point.
Dunbar's number makes little sense for directly determining/classifying organizational sizes. Most of peoples' relationships lie outside of the organization they work for. You'd probably have little to no relationship with people in your organization way before you reach Dunbar's number.
But yes. 150 is far too high a number. I'd say it should be closer to 50.
It makes me sad to link to this, since the PARC blog no longer hosts his images, but it's more than nothing: http://www.lifewithalacrity.com/2005/10/dunbar_group_co.html