I believe that I'd do that deal.... It would help if we knew the rest of your line-up and your league's scoring system, but I have Boldin on one of my teams and he has underperformed thusfar, but I expect him to have a better 2nd half of the season....
Warner is an upgrade over Tony Homo.... The biggest gain in my opinion is Chris Johnson. As bad as the Titans suck, he is a Top 5 RB and will continue to be the focal point of the Titans offense. He's the only player on the Titans that I'd even consider rostering right now.
If I were trying to compare, I'd look at it this way....
Boldin <<<<< Moss
Warner>>>>>>>Hightower
Chris Johnson = MJD
M Forte = Portis
...but this all depends on the rest of your roster, your league's scoring system (PPR?) and your needs/strengths/weaknesses.... This looks to be a fairly even trade. Just depends on what you think these guys will do in the second half of the season....