Granted the baron is not exactly a speadster when it comes to climbing. Check your prop Pitch settings and make sure they are at max. Also check out how much fuel you have on board and any extra weight. The service ceiling rating for any air plane is under ideal conditions no head or cross wind and after 3:00 on friday Dont forget to check that barometic pressure as this goes down so does your service ceiling Make sure your deice is on. You should be able to make this altitude I fly the default baron at that altitude all the time. Try starting at a lower altitude say 10,000 feet let the plane pick up air speed then go up another 2,000 feet. Sometimes this is the only way you can get there. 8)

The reason the Baron is a dog is that G-Max doesn't simulate true performance.  Sim aircraft are only relatively different.  You may find Baron performance specs on the net.  Search Google.  Quick and dirty: Set both Parasite_drag_scalar and Induced_drag_scalar to 0.6 in the aircraft configuration file.

Guru I don't know what your post means.
Here is a similar Baron performance complaint a different forum:

"I read in the Microsoft documentation that the service ceiling for the Baron 58 is right at 20,000 ft. No matter what I do, I can't get the Baron much above 12,000. I did finally get to 15,000, but the Baron had slowed to near stall speed, and was in an emphasised nose-up attitude.
Yes, I have flaps and gear retracted. Yes, I am leaning it out to max RPM. Am also making the necessary adjustments to the props. I'm taking off from near sea level with mild temperatures, so density altitude is near actual altitude. Have even adjusted cowl flaps to optimize engine operating temperature.
I have not experienced such poor performace vs specs for any other FS9 aircraft.
Anyone got any tips?"

Here is the Raytheon specs for the aircraft:

http://www.raytheonaircraft.com/beechcraft/baron/Baron58_Spec_Perf_2003.pdf

Problem is that the COF version will not climb at 300 fpm at 16k.   I'm surprised that it climbed to 16k based on my few flights.  Service ceiling of 20k is the altitude at which the aircraft will climb at 100 fpm.  Likely at very low fuel weight.  The engines are unboosted and that means best cruise is much lower than 20k.

I know from flight tests of AvHistory 3 series fighters, also G-Max, that they do not come close to published AAF and manufactuer climb specs.  Level speed mark at 10k is reasonable but climb isn't close to the real deal.