A point of correction. The limited Crossfire only has 215 hp, not 315, so if the 3.7 is over 300 hp then a swap would have significant increases.
The SRT6 has 330 from the factory so if you are comparing an SRT6 to the 3.7 then I'd say you are correct, a swap makes no sense. The SRT6 hp, while downgraded from the merc AMG SLK version, didn't actually mean it the SRT6 didn't the the HP or the potential for the HP that the AMG had. Many say it was just a paper reduction and not an actual reduction. Anyway, seems your options are to buy an SRT6, or an upgrade would be in the cards if you are really looking at upgrading a limited, which only has 215 from the factory.
Have fun.
Good points. You beat me to the correction on the limited @ 215 HP.
Yes, many believe it was a paper change on the AMG/SRT as well as the SLK320 @ 225 HP and the Crossfire Limited at 215 HP. Which lays questions on the mathematical projections often made....including mine.
I agree with your premise here, a swap makes no sense. Especially given the proven potential of the existing power plant in the SRT. But for the right guy wanting to do something different, have at it.