There are several factors that effect boiling temperature.
Added Sugar:
Assuming your Christmas Ale has a SG of 1.075. The increase in temperature due to added sugars will be on the order of 1.5°F.
This increase in temp is due to the molality of the sugars in the wort. An SG of 1.075 is approximately equivalent to a molality of 1.1; assuming the best case of all sugars being glucose. Any maltose, longer chain sugars, residual starches or proteins will reduce the over all molality of the solution and thereby reduce the increase in boiling temp.
See boiling point elevation.
http://www.chem.purdue.edu/gchelp/solutions/eboil.html
Hydrogen bonds and proteins:
These sugars (whatever their kind) as well as any proteins have an additional effect on the boiling point. Glucose, maltose, etc. being composed of carbon, hydrogen and oxygen will bind chemically with water via hydrogen bonding. Proteins are large molecules with many hydrogen atoms. Water can bond to proteins via hydrogen bonding as well. In addition to this, water may become trapped in the tangled mess that make up many proteins. The result of these reactions and trapping is to effectively increase the molality of the solution as there is less free water to boil. I found no definitive data but my initial evaluation indicates that the total effect will be an additional increase in the boiling point of <1°F
Position of the Thermometer:
You do not describe the configuration of your boil kettle or boil size so I will make some assumptions.
6.5 gallon boil volume
10 gallon BK
Thermocouple installed in the side wall ~ 6" from the bottom
Rolling boil
Temperature Gradient
The temperature
of the bottom of the BK (not of the wort) will be substantially above 212°F. I have not had time to work through the heat transfer equations yet but I expect it to be in the 250°F-300°F range. There will be a temperature gradient between the bottom of the BK and the top surface of the wort. Due to convection and a rolling boil this gradient will be relatively small but will effect an increase in the reading of the thermocouple. Say +1°F
Head pressure:
Wort will be boiling on the bottom of the BK. Due to the height of the wort in the BK the boiling point at the bottom will be several °F higher than the boiling point at the top. 18-24 inches of wort will increase the BP from 212°F to ~216°F. With the thermocouple 6" higher the boiling point will be ~215°F. +3°F (This does not take into account the reduction in BP due to your elevation as noted in a previous post.)
Superheated vapor:
Since the wort is boiling off of a ~250°F surface the gas in the bubbles will be hotter than the surrounding liquid. As these gas bubble pass by the thermocouple it will register a higher temp. Due to the short resident time and the small heat transfer coefficient of the water vapor this will probably have little effect. +1°F
Summary
+1.5°F for sugars
+0.5°F for trapped water and hydrogen bonding
+1.0°F for temp gradient
+3.0°F for head pressure
+1.0°F for superheated vapor
=====
7.0°F total or ~219°F.
Granted I made several assumptions and it has been a while since my thermo and chem classes but I think that the major increase in measured BP is due to the position of the thermocouple. The increase due to SG alone is not sufficient to account for the 8°F+ change you noted. Anyone experienced in thermodynamics is welcome to correct any of this.
To achieve this temp increase only due to SG we would need an OG of ~1.600 or ~84# DME for a 6.5 gal boil volume.
YMMV
