HC is slower than other stem plants, but once it gets established you should be able to see the new growth. I do think that trimming/topping the HC will help it to send out side shoots and spread quicker. Adding excel also really gives the HC a boost. I have pressurised co2, but definately see a growth spurt when I add excel to the tank as well.
One thing to remember, when you do get your HC growing, dont let it get too thick. Mow it down occasionally. I just learned this the hard way. My foreground was about 1.5" thick and the bottom of the mat of HC started to die and melt away. Part of it started to rise off of the substrate. I had little strands of HC breaking off and floating on top of the water. By the time this started happening, it was too late to just trim it down. I had to pull up the entire lawn of HC, in one big mat, and replant the entire foreground, strand by strand. I hated to do it, because it looked so good and took so long to get that full and thick, but it was just going to get worse if I didn't do something about it. So... just keep an eye on it further down the road.
4 watts per gallon is very high light intensity for a 55 gallon tank. And 8 watts per gallon is a getting to be pretty high light intensity even on a 10 gallon tank. It isn't too much if you mean it will kill the plants or fish. But, if you don't really keep up with the maintenance of the tank, prune every other day, make sure the CO2 concentration in the tank stays the same day after day and is about 30 ppm, and you make sure you are dosing every nutrient so as to avoid a shortage of any nutrient, the plants will not grow fast and algae will soon take over.