I didn't do so hot on my first attempt and died rather early. I wasted my time growing the plant.
But the second attempt was a rather easy, as I made sure to spend those precious seconds at the beginning building up my research speed, followed by reload speed and cash bonus. I found Fast reloading is the key to getting any research done at all in some of the rounds (that and having a big enough gun due to some fast cash, of course). Losing an advantage in any of those categories seemed like it would be a real struggle to catch back up. The rest of the upgrades I just filled in after I maxed out research, reload, and cash.
I did find that a 5 for plant growing still took multiple rounds to grow, which was kind of boring when I'm mowing down everything with a heavy machine gun and then just standing there for a minute before repeating.