'Is such a high growth rate (assuming that everything is optimal) actually possible[...]?'
It sure is. Yeast will pretty much grow indefinitely as long as there are no nutrient limitations or inhibitory levels of ethanol. The major limiting nutrients are:
- Fermentable sugar
- Nitrogen (amino acids, basically)
- Oxygen (more accurately, the lipids synthesized in the presence of oxygen)
- Vitamins and other micronutrients.
In a regular batch fermentation, the limiting factor will usually be sugar, but may also be oxygen (a 'stuck' fermentation). Nitrogen generally isn't limiting in all-malt worts, though it can be when high levels of adjuncts are used, or for very high gravity worts. Zinc is the only micronutrient limitation you'll likely encounter.
In a stirred starter, it will most likely be sugar (again) but could be nitrogen, especially if you use simple sugars to fortify the starter wort, or, again, zinc.
As reported in Brewing Yeast and Fermentation:
'Typically, brewing yeast growing on high-gravity (15-18°Plato) wort under continuously aerobic conditions [...] yield a terminal yeast count of 200-300 x 106 cells ml-1'
In contrast, given your situation, 450 x 109 cells per 3 liters (or 3 x 103 ml.) leaves you with 150 x 106 cells/ml., only as little as half as a typical number (as quoted). Of course, this is easily accounted for by the fact that you are using a 10°Plato wort, but by extrapolation you might expect to be able to get the same cell count from a wort as low as a 7.5°Plato.
So, if all you cared about were cell count, you could hypothetically push this pretty far, as the extent of overall yeast growth will generally be only limited by wort strength. However, as I mentioned above, alcohol levels become damaging to yeast at a certain point, so you generally don't want to go too high (even 18 Plato seems a bit high to me).
But, in essence, yes this is an entirely feasible level of growth, given the right conditions.
