Yes, you are correct. I'll try to be as succinct as possible:
A full exhaust kit, when proper R&D is applied, will provide a larger gain than a slip-on. This is especially true when the stock piping is restrictive. However, with full systems that use large diameter pipes, it's quite often the case that high-rpm performance is increased at the expense of low-end torque. It's also more expensive than a slip-on, and sometimes the difference is negligible in real-world riding, making it unworthy of the cost.
Ask yourself: which do I value more, low-end power for stop-and-go riding, or high-end power for all out racetrack performance? Is it even worth spending hundreds of dollars for a minute power gain?
Loosely speaking, less restriction = more power. Of course, things are more complicated than that; you should Google "exhaust back-pressure" or "exhaust scavenging" if you're interested in the topic. Car resources are fine because the physics are all the same.
There are also issues of fuel mapping and tuning, which exponentially complicate things. There are forum threads about resetting the ECU, though I'm not sure if a consensus has been reached. You should keep in mind that Honda does more research in developing a balance in exhaust performance, whereas an aftermarket product may not be so carefully engineered.
I went with a GP-style slip-on (M4 Street Slayer), more for the weight savings than anything else. I was shocked to feel how heavy the stock muffler was.
This post was brought to you by our sponsor: me being bored at work.
Poly's post sounds reasonably well-informed, and my comparison of Dyno charts from the 2 Brothers website backs him up. I bring up 2 Brothers as they are it seems one of the more well-regarded exhaust manufacturers to offer both full exhaust and slip-on systems for the CBR250R as well as honest-seeming dyno charts for all of those configurations.
I'll be getting a slip-on of some sort when I'm comfortable doing so (probably after christmas having chased down the last 100 or so miles to my 600 mile initial tune-up) but as Poly said mostly for the weight savings and without expecting much more then an extra horse and a half. Just a little boost to help me reach cruising speed on the highway in a more comfortable fashion, all I really need.