As an owner of the 2015 Cannondale Quick CX 4, I think I can speak from experience on this bike quite well.
Firstly, if you're looking to change to slicks, 700 x 28c is a great choice. The rims feet this size quite well. I've been riding for about 2 months on a currently 7 mile commute each way. Got my average speed to usually 14 mph if I'm pushing. With the 28s, I'm easily at high 15s, and 17-18 if I'm pushing. There
is a good benefit from going with thinner slicks. For me, I chose to get some cheap Continental Forte Strada K's. Once they wear out, I'll get the Continental Gatorskins or Schwalbe Marathons. And of course, my Happy Mediums are still there if I ever want to hit the fire roads.
Secondly... you'll get faster in time as you ride more. I'm starting to see that the "engine" upgrade that everyone talks about is solid. Keep that up.
Third. You can make your bike just a bit more aggressive by messing around with the fit of the bike. For example, make sure your seat height is high enough that your legs are just almost fully extended on the bottom most point of your downstroke, but not so much that your hips are moving side to side. I'm 5'11" and got a Large frame, and after riding the bike for a couple weeks, I just noticed that my thighs were much too sore to keep going. Adjusting the saddle helped tons.
Also, you can play with stem height. I reversed my stem so that it doesn't angle upwards, but downwards. Also I moved the spacers so I have two spacers on top, and two more on the bottom. This gives me a more aggressive position.
With these adjustments, I'm starting to feel much more content with my purchase. Try some of those out, you might too.
Here's my rig:
Threw some bar ends on it, wrapped tape on it, and it gets me low enough. Not drops obviously, but definitely comfortable and lower.