Hi,
I'd put a little effort into collimating your dob. I like star tests as the final step after lasers, cat's-eyes etc. Here's a good read
http://legault.perso.sfr.fr/collim.html
However there are many strong opinions on the best collimation methodology.
Then, a decent EP makes a lot of difference, even in an ordinary scope. I recall once with a SW102 refractor, looking at Albireo, trying first a Baader Hyperion (not a rubbish EP at all), then a new Teleview Ethos. The difference was so great I was gobsmacked.
Now, you don't have to rush off and spend $800 on one EP, there is good glass for less, especially here in IIS classifieds. Maybe a good second-hand EP or 2 there?
Magnification, well, as with so much other astro, less is more. I think that going for x400 or whatever is more trouble than it is worth. Image quality deteriorates, it jumps around in the FOV, and the object becomes more difficult to get centered. It is a matter of preference I believe.
The most influential thing I think is the one least in your control - the seeing. If you get a steady crystal clear night you may be able to split Sirius!
Cheers