Re: What would happen to relativistic mass if slowed down?

Your question illustrates the confusion that can arise when the term
"relativistic mass" is used.  Although popular in the early days of
relativity, and still used by some physicists today, the concept of 
relativistic mass has been discarded by most theoretical physicists.
The mass, m, is considered to be an invariant intrinisic property of any
object.  In terms of the mass, the energy of an object is given by 
E=mc^2/sqrt{1-v^2/c^2}, and its kinetic energy by T=E-mc^2.
Just as for the nonrelativistic limit, the kinetic energy depends on the
velocity of the object, but its mass does not change.  In your example, the
reasoning is no different than it would be if T=mv^2/2.  It is just the
explicit form of the equation for T in terms of v that changes.