Pretty neat so far! It seems a shame to be wasting a lot of the emitted light from the top of the bulb. Maybe in time a idea will come along as to how you can direct the unused light more out and downward. Maybe the whitewash will reflect/defuse a lot of it back down though, we'll see. You know if you think about it, the only real light you're utilizing is only that small portion that strikes the leaf--the rest is wasted. And that's true no matter what kind of set up you have, whether it's a T5 like mine or any other. You'll have to keep a close eye on temperature until you gain some experience with your setup because this is a pretty high wattage bulb. I've found that in general, cooler conditions with lots of light produced the best bulb growth.
I don't have any experience with whitewash. I would go with all the natural light I could get, but stick with your friends advise for now because he's familiar with whitewash and I'm not.
I think your are going to be pleasantly surprized at how well this is going to work.
