All right, lets see if someone....besides DanQ :lol ... can explain what happened here.
The back part of the groove has gone inward as you can see me pointing it out in one of the pictures.
Well, you tires are more of a case study, and we've had a lot of talks about them. The main points of focus are:
- 5 year old shock absorber
- weight outside of the spring/damping ideal weight
- you are getting fast as snot, and need some adjustability
The problem with the age of the shock is in the fact that with 5 year old oil, there can never be 100% accurate damping. The damping speeds will be inconsistent throughout the travel range as the shock moves. This alone can have the rear fighting for traction. This can be fixed with a simple service with new fluid, seals, bushings etc (typically around $150 or so). New oil = new life.
The weight outside the spring and damping range. This is going to cause the damping speeds to again be outside of ideal, with some of the adjustments at or close to maximum setting. Now combined with the age, this is an easy fix, adding about $100 for a spring and about $200(ish) for custom valving, to the service listed above and you have a fresh shock, properly sprung and damped for you. This is a really good approach and would make for a good shock absorber within the adjustment capabilities of the stock shock, with custom tuning for your weight and riding preferences.
With you becoming faster, there are multiple issues. Your traction needs are going to increase, as well as lean angles and corner entrance/exit speeds. The damping requirements for a novice are going to be very different than an expert. An aftermarket shock would be able to handle a wider range of riding demands, with features like high speed compression damping, adjustable ride height combined with a spring and damping curve set for the riders weigh and preferences.
In my opinion, the advantage of a replacement shock in your case is primarily the ride height adjustment. While ride height can be changed by shimming the shock (it's been done that way for years) it's way easier and more granular with an adjustment for ride height. Combined with high speed damping it will give you the needed adjustment range to keep your tire in contact with the ground for most conditions you'll encounter between services (about once every year or two).