pitch 150ml of thick slurry there is no need to use a starter since all of the needed cells should be present?
Yes and no. The starter not only increases cell count, but it also "proofs" the yeast. If you familiar with baking, making bread in partcular, its common to throw the yeast into water with some sugar to proof it. It lets you know the yeast is viable. The same idea applies to a starter for beer. Your proofing the yeast as well as increase cell count.
Having said all that I've used 3 week old yeast off a batch that I washed. Didn't use a starter and didn't have any problems. If it was a recent harvest of yeast and you had a healthy fermentation, and clean beer from it you can probably just pitch at the rates on Mr Malty, but at some point the amount of yeast you need to pitch is stupid. Play with the dates on the calculator. I think I set it to 2 months old and it said I needed close to 1000ml of yeast.
Yes pitch rates make a huge difference in the beer with out a doubt.