This idea of holistic tuning can be applied to the entire car. Increasing power and torque requires improvements to suspension and braking systems. a larger turbo or supercharger will show up the vehicle's other systems as inadequate. Larger contact patches will be needed to get that extra power to the ground. A recalibrated suspension will be needed to deal with the added forces of more mechanical grip. Braking capacity will need to be increased to deal with the extra kinetic energy created by greater straight-line speed. Pretty soon, the 'stealthiness' of the modifications has all but disappeared, giving way to larger wheels, lowered ride height and visible add-ons.
Some enthusiasts have found a work-around with a tuning style known as OEM Plus, using factory components from other models or even stock parts sourced from foreign markets that still have a factory appearance. Volkswagen enthusiasts have been swapping K04 turbos and associated fueling components from 225-hp TTs on their 1.8Ts with great success. BMW E36 M3 owners source the more powerful Germanmarket M3 powerplant for big gains. Some even turn to factory performance parts available from the dealership. Outfits like Volkswagen's Driver Gear sell everything from sport springs to exhaust systems, all backed with a factory warranty.
Older vehicles have been fair game for sleeper projects for years. Audi tuners have been swapping later and/ or larger engines almost since the beginning of the brand. VW enthusiasts started swapping 16-valve motors into Mk I cars, then later turned their attention to swapping VR6s into Mk IIs, and more recently 1.8Ts into anything and everything. Even Porsche enthusiasts (who are favored by modular engine designs) have taken to swapping entire engines and electronics from newer vehicles. Sometimes advances made by the factory are the best and easiest solutions.
The future looks even bleaker than the present for sleepers. Manufacturers are becoming ever more diligent about making life harder for tuners. Automotive ECUs are increasingly difficult to access and legal steps are being taken to discourage tuners from tinkering with software. As I write, BMW is fighting tooth and nail to keep tuners out of the new twin-turbo six's ECU. The thought of easy horsepower drove initial sales of the 335i. Most enthusiasts believed 50 or more horsepower would be just an ECU flash away, but sadly that has not been the case. The best software tuners will always find a way to get into new ECUs, but a flash will likely become more expensive and will need to be purchased as a part of a tuning package, not as a standalone upgrade.
Hard parts will always be around, but as stated, they won't supply such a large improvement without software cooperation. But the real downfall of the hardware tuner may end up being environmental laws. CARB certification is difficult and expensive to get and only the most diligent tuners are willing to undergo the process. As more states adopt CARB standards and increase yearly inspections, it will be harder for enthusiasts to modify their vehicles.
All these things may combine to become the ultimate downfall of the sleeper. Most drivers aren't interested in blending in any more. Manufacturers do everything they can to get their cars noticed. Changing views, along with the difficulty in creating a homemade sleeper, may lead to a downturn in projects. Rising fuel costs and environmental concerns may lead to a sudden revelation of: 'we already have more power than we need.' Once power starts declining, the sleepers will rise again.