Yo Dave: I have rifles in 444 with 3 different barrel lengths: 26", 20", and 18" - ported. You won't loose that much velocity from the 20" to the 18" to make it an issue. However, if you intend to reduce the length of a 444 Marlin caliber rifle to 16", you are settin' yourself up for a bunch of shoulder punishment. My 20 incher kicks harder than my ported 18", si I can just imagine what an unported 16 incher will recoil like.
You might wish to consider one of the ported shorter factory productions, either the Winchester Big Bore Timber Carbine in 444 (although they are now available, supposedly, in 450 and 480 calibers) or the Marlin Outfitter and add some csutom accesories to either of those. Either should be fairly easy to bring back to target quickly, where I think that your proposed 16.5 incher will be more than just a bit on the wild side. But, this is just my 2 cents worth and I HTH. Mikey.