Thermal Rash Guard or Vest?

I was thinking of buying a vest and fitting it inside my wetsuit for additional warmth.
However, I pulled up the oneil site and noticed they carry thermal rash guards. Have any of you tried one or similar?
http://www.oneill.com/#/men/americac..._crew_1/black/
The one they call thermal is 8oz, the thicker ones I own are 6oz. My other rash guards, if I had to guess, seem like 2oz or 3oz by comparison.
What do you use for additional warmth under your wettie?
However, I pulled up the oneil site and noticed they carry thermal rash guards. Have any of you tried one or similar?
http://www.oneill.com/#/men/americac..._crew_1/black/
The one they call thermal is 8oz, the thicker ones I own are 6oz. My other rash guards, if I had to guess, seem like 2oz or 3oz by comparison.
What do you use for additional warmth under your wettie?