Even though the concept is moot, here are some real sleeper mods:
Front axle:
Beef Tubes, Overdrive gears, CVDs, Stronger steering link (the GPM above is fine), no slop locker
Rear axle:
HD gears, no slop locker
Suspension:
Pen spring inside the shock body under the piston to reduce ride height and control down travel.
Delrin or Aluminum upper links (again, no one cares that you got new links) if you can drill out the plastic links, you can already make your own delrin links.
Wheels:
Throw some weight in there, but no need to go crazy.
Tires:
Stock trepadors or ripsaws are fine and fit the sleeper look.
Transmission:
Metal gears/steel outputs
Play around with spur/pinion combo with the motor you get to find what you like
Flip/mirror trans to get the motor to point forward (can't do this on some bodies...)
Electronics:
Kit versions don't come with them, so there's nothing to hide. Get the good stuff. At least 250 oz-in servo, Sidewinder/Mamba/BRXL/FXR ESC, 35-55T Crawler motor, etc. No need to re-sticker a sealed can motor (you probably wouldn't be able to do it clean enough anyway, so no need trying to waste your time.)
Move the battery to the front (this is a stock option on the newer kits with instructions on what parts you will need), and go 3S LiPo (only if you go to a different ESC capable of 3S).
Everything above is pretty low-key, but actually adds performance. If all you want is to trick your friends, I refer back to the link I posted previously about the magic kit you can buy. There's gotta be like 100 tricks you can show them.
You don't need to drop a bunch of money to have fun. I've always been into budget-minded mods, and love to see the innovation that breeds. I think sharky's intention was in the right place (innovation maybe?), but maybe the message could have been delivered differently.
Less "hey you can fool everyone into thinking you have a stock rig that performs better than stock, making people think you are actually the cause of the increase, or that it is some magic rig" and more "simple unseen modifications to retain the stock look" would have been my suggested approach.