I have shrouded down quite a bit in both hot zones and non-hot zones. What I have seen, but cant quite prove, is that the exp I get while shrouded is the same exp quantity for the mob for that level but since it cons blue/yellow/red you actually earn it, where you would normally not.
My lvl 72 warrior alt shrouded to lvl 50 to 3-box in Sebilis with a 47 necro alt. The necro alt gained levels normally with normal exp in a hot zone. The 50 (bear-warrior) got exp on all the same kills since the two were grouped together. As I periodically checked Magelo, (earned exp is not displayed while shrouded), I saw that my 72 war earned 2-3 blues of normal exp in the same time my 47 necro got to lvl 53 or so.
The next day I shrouded the warrior to 55 bear and fought juggs and reets for more exp. Magelo showed he earned 1 yellow of exp more into lvl 72 while the necro leveled to 59.
So my thinking is that exp earned while shrouded equals the same quantity, or points if you will, that you earn at that level. But since at lvl 72 you need more exp points to reach 73 than you need at 47 to get to 48, it just appears that you earned less.
The great thing is that you do actually earn exp even though its a relatively small amount. Had I not shrouded the 72 war, he could not have earned exp and it would have taken much longer to solo the 47 necro in sebilis.
The real question you have to ask yourself is: Will you double your kill-ability by adding a shrouded player or is it just a small increase over soloing? If you can solo fine then you will earn more exp than grouping with a shrouded player for faster killing.
For me, having the bear to tank everything while the necro was on pure DPS (their specialty), more than compensated for the exp split for not soloing. I just had the bard outside of the group singing regen songs for them.