I remember reading this year or last year that the 600 and 65 got a small boost in power.

According to the MB site the 0-60 is now 4.2 and used to be 4.5.

any idea if this happened 06-07 or 07-08?

I never heard the 65 got a boost but I think the 600 did as well as the 55. I am guessing 07's.

I never heard of the 65 getting a boost either

I think the power rating stayed the same through all the production years.

That was only because MB decided to give a bit more realistic numbers. The power and cars stayed the same, they just changed their recorded 0-60 numbers.