6 Month update: Overall, overkill. For a few dollars and hours' time, it was worth it while being still ungainly, ugly and heavy. Much of the desired stiffening has been achieved with new designs (ie I3 Mk3) but if you don't want the money layout required for extrusion channeling, or a new printer, additional bracing with M8 rods and secured footers is helpful. The paver's sound dampening brought noise levels down far enough so I could print overnight without being awakened from a few rooms away (I'm fairly sensitive to intermittent noises). Quieter fans and an enclosure with sound blanket took care of the rest.
I liked aspects of the various Prusa I3 Mk2 frame braces on Thingiverse so I made my own in Fusion 360. In conjunction with a z-frame brace https://www.thingiverse.com/thing:1895661, x-frame brace https://www.thingiverse.com/thing:2438948 and TPU booties the printer is incredibly rigid and quiet. I believe this will improve high speed prints and has improved acoustics.
Update: After upgrading my Mk2s with frame braces my printing quality has improved markedly with both 400u and 250u brass nozzles in PLA. Printing at 40mm/s and 25mm/s Slic3r Prusa Edition perimeter default speeds respectively, .75mm dia meshmixer supports are actually printed successfully and cleanly removed from model and there's been less hairs and stringing on models overall. I can't tell if overall dimensional accuracy has improved but with less material needed and post-processing time required, I'm very happy. I can't tell a difference with ABS because I didn't print with it much before upgrade but the 400u prints so far have been very clean prints (so long as I use sufficient numbers of supports for the overhang).
Note before beginning: I removed the PSU and modified the Rambo case. You don't have to modify the Rambo case, just move that leg back a bit more. I moved the PSU out of the enclosure b/c I print with ABS as well.
2x 400mm 5/16" or M8 threaded rods
8x 5/16" or M8 nuts
6x washers 9mm ID/18mm OD
4x 18mm M3 bolts
4x M3 nuts
4x 6-32 or M3.5 bolts and washers or screws to secure lower brace to base.
1x rigid base at least 400 mm x 400 mm. Concrete paver or 1/2"+ MDF board.
This rear brace is designed to be secured to a base independent of the printer frame so vibrations are not transferred directly to the rest of the printer. I used a concrete paver for more vibration dampening https://www.youtube.com/watch?v=OnfYA5QLA84 and rigidity.
The lower brace uses a captured bottom nut. I used the threaded rod and a nut and washer on top of the lower brace to screw the bottom nut into the lower brace's cavity till it's bottomed out. If it's too tight (for an M8 nut), simply enlarge the lower brace a few %. The rod bottom should be flush with the bottom nut's bottom so the lower brace can sit flush on the base.
The upper brace will need supports when printing on its side but I left them oriented that way to transfer stresses from the frame to the M8 rod via filament strands instead of filament bonding layers. I used the extra 18mm-20mm M3 bolts and nuts from the MK2S kit to secure the upper brace to the frame. Simply snip any extra material away if you're using larger bolts or nuts.
Assembly: Remove PSU. Assemble lower brace on one end of threaded rod. Emplace upper brace (designed for upper z-axis screw holes) to z frame. Thread a nut and washer on free end of the threaded rod before inserting into the upper brace. Test fit positions and clearances. The back of the lower brace is designed to be 170mm from the Z-axis frame. Mark positions and drill holes. I used epoxy anchored 35mm 6-32 bolts in 1/4" holes sunk in a 400x400x40mm concrete paver to secure the Z-frame braces, lower braces and x-axis braces (sequentially in that order). A 1/2"+ board of plywood or MDF with wood screws would work too (and be a bit easier) but I think the paver was worth it. Install vibration feeties to printer frame feet if so inclined.
After securing the lower brace to the base, lightly finger tighten the top upper brace nut and washer so it's snug but doesn't deflect the z-frame. Tighten bottom upper brace nut and washer. In conjunction with already installed z-frame brace, the frame should be super rigid now. If no z-frame brace, x-axis braces would secure the printer frame to the base creating a rigid structure as well.
I used some TPU flexible feeties with .45 mm thickness on the printer's 4 x/y axis feet to equal the z-Frame brace's .45 mm offset ( I printed with 3x .15mm layers). Also made some flexible anti-vibration pads for the paver for better vibration transmission isolation. Overall leveling achieved with cardboard scraps under anti-vibration pads.