1st off, capacity plates really don't apply to boats > 20' as they are not mandated by law pursuant to 33CFR, Section 183 [as per USCG Federal Boat Safety Act of 1971] which details the weight & HP capacities of 'small' boats.
That said, while it's not a Federal violation to exceed the listed capacities, your state might have a statute prohibiting exceeding the stated capacity. Or even the installation of a motor that exceeds the recommended horse power limit. For you personally, you could void your insurance policy if you're found to have exceeded the limits stated on your capacity plate.
FWIW the USCG has recently changed or updated the average weight (Americans are getting fatter ...) where the average person was assumed before to be 150-pounds ... I now believe it is 180. So my '92 2520 that wears a plate for 12 would nowadays carry a plate for 10.
Per Boat/US, for boats over 26 feet, a general way to determine how many people you may safely carry is to use the following formula:
Boat length (in feet) multiplied by Beam (in feet) divided by 15 (constant)
Note that is based on the old average of 150-pounds per person. They also caution:
Also keep in mind the limits defined on capacity plates apply in good to moderate weather conditions. In rough waters, keep the weight well below the limit. In ideal weather conditions, it is recommended that you distribute your load evenly, keep weight low, and avoid abrupt changes in distribution. This is especially important if your boat’s capacity is fully used or if weather conditions deteriorate. It is also important to remember that people represent a "live" load and moving about affects a boat quite differently than static loads. In general, shift human or other weight only after stopping or slowing. Keep in mind that this formula is based upon having calm seas! You should always carry less weight and fewer people if you know you are in for rough weather.