I don't think the zoomed dock is harming his plastron. Lots of us use it and the turtles do fine on it.
Keep an eye on his plastron and if the color changes or gets brighter, let us know. Since I haven't actually seen a picture of it, it's possible I'm misunderstanding the situation but it doesn't sound bad from what you've described.
How much does he bask? When he basks, does he dry out completely - top and bottom? What kind of UVB light do you use?
On his size, have you actually measured him? When you measure a turtle, you're supposed to measure the "straight shell length", in other words, the length of the shell from front to back without taking into account the curve in the shell. If you haven't actually measured him lately, why don't you do so in the next day or two and let us know what you get.
Have you seen this info on nuitrition? It's great - it tells exactly what to do.
http://www.redearslider.com/index_nutrition.html
On how many pellets to feed your turtle, it's supposed to be as many pellets as would fit in his head if his head were hollow. Here's a cute thread with photos on how to determine the right amount. I think the turtle in the photos is supposed to get 4 to 6 of the pellets every day. You don't want to overfeed your turtle, of course, but you don't want to underfeed either:
http://www.redearslider.com/forum/viewtopic.php?t=11368