All timing belts are the same length, and for a reason. As I recall, one revolution of the belt will result in all sprockets being one tooth different from where it started. In the event a sprocket tooth has a shape problem or deris or such, issues from abrasion will be mitigated by spreading over as much of the belt as possible.
There are adjustable centerline cam gears, but not any that I'm aware of that also utilize AVCS. Eccentrics is how it can be adjusted for decking, since it affects both intake and exhaust.
If you are competant in tuning, you could use your AVCS maps to adjust centerline, but you'd need to degree beforehand to know how much you needed. Also keeping in mind that AVCS maps are in crank degrees (double cam degrees). For example, if you needed to retard 3 cam degrees, you'd run one tooth retarded (15 crank degrees --> 7.5 cam degrees), then advance AVCS 9 degrees in all cells and tune from there. This potentially limits your max advance, since the stock USDM ECUs seem to limit at 25 degrees. Exhaust is opposite direction in AVCS maps if you have an 08+STi... you could use static adjustable cam gears for single AVCS cars.
HOWEVER, timing marks don't always line up on a freshly installed belt due to tensioner not settled yet. Try (after releasing the tensioner) turning the crank CCW 90, then CW 180, then CCW 90 to get back to the marks. That may bring the marks closer (unless it's decked).