Your G8s are probably 120VAC, right ? As such, you are not going to replace with LED without switching to LOW DC voltage. Most are 24V.
If you want dimmable LED (a good feature), you'll need a dimmable constant current driver and a magnetic transformer and a magnetic rated dimmer. You can use a standard dimmer, but the current surge will burn it out. (ask me why I know...)
Bottom line.. rip out the sockets.
Not a complete answer, but here's some more info.
Brightness
While there are ways to calculate this that aren't that complicated, it requires knowing many coefficients that are not readily available, so ends up being an educated guess at best. You can do just as well totaling your current lighting's lumen output, then adjusting for changes in fixture styles. (I consider even a simple LED strip a "fixture" for the sake of this discussion) It's a starting point. If you use proper task lighting, there's a lot of room for variance in total levels.
Uneven light
You do not want to see the actual LED elements, it's really annoying. Besides diffusers, indirect lighting techniques work well, though require more lumens due to losses through reflection.
Positioning
I'm not sure what the issue is with parallel surfaces. The efficiency is mostly inherent in the fixture's ability to direct light where it's needed. The mounting surface has little to do with this, you need to direct the light where it's needed.
White Light
The "whiteness" is measured in degrees Kelvin. The temperature a black body must be heated to emit the same color of light. Standard daylight is often taken to be 6500K, anywhere in the 5000-7000 range is close enough for most tasks. You eyes adapt quickly to small variances. In residential settings, daylight is considered too harsh because we are so accustomed to incandescent lighting, so most home lighting is produced to output more in the 2000-3000K range to mimic the orange incandescent lighting.
Noise
How audible the noise is varies by manufacture and environment, as well as personal temperament. I don't think noise data is readily available. Testing in a noisy store will not yield any useful data, other than if you can hear it from a few feet away it is too loud. Noise can be attenuated by placing it in an enclosure, but keep in mind power supplies generate heat that has to be removed by ventilation, you cannot seal it into a sound proof box.
Try to place power supplies as far from your pillow as possible, trying to sleep is when it will be most bothersome. Placing it lower so there is likely to be more furnishings obstructing a direct path will help, as well as being surrounded by soft materials that do not reflect sound as well.
Best Answer
The RGB controller's output is only relevant to the 3-color LED section. Per 60 LEDs, they use 0.4A per channel (1.2A total). 48 watts is 4 amps.
So if your strip is less than 10 feet long you should be within limit of that controller.
If you are near or exceed the limits of that controller, you can add a device called an amplifier in between the controller and the LEDs. The same controller can power several amplifiers if needed. At that point, the controller's output is no longer used for power, but merely as a signal to the amplifiers. So if the signal has some distance to go, it can travel on thin thermostat cable instead of heavy cable.
You will not be able to use the RGB controller to dim the monochrome strips. It will not do what a person expects, unless they know a lot about RGB blending, which your guests won't.
Also keep in mind these cheapie LED dimmers are not intended for use as on/off switches, and will leave components energized even in the "full off" position. Not something you want to do. Aside from being a fire risk, it will be a considerable vampire load 24x7, which will drive up your energy costs.
The better way to think of dimming is as a fine adjustment after the switch.