I'm not 100% sure, but I think "book spec" HP ratings are from just the engine, on a test stand. You are probably right that once installed, the actually yield will be a bit lower.
As far as cruise speeds are concerned, unless an altitude is specified, they probably go by TAS at whatever altitude gives the best airspeed for that engine/prop combo on that particular airplane. Based on the performance charts of other aircraft I've seen, these figures, like those for takeoff distance, etc. are given assuming max gross takeoff weight.
You will also find that cruise performance charts show airspeed and fuel consumption figures based on specific percentages of available power... for light singles, it's usually 60-75%, but sometimes a number of specific percentages are shown.
But I'm no expert... I'd suggest you ask Cessna, or try a Cessna owner's forum.
Or you could try this forum... not specifically for Cessna owners, but there is a wealth of knowledge on the boards... some members are also aeronautical engineers and mechanics (or just know-it-alls, LOL).
http://www.pilotsofamerica.com/forum/
by patchz » Fri Jun 17, 2011 7:48 am 
