"My son is shooting a standard barrel, and then the following year the light weight barrel came out. His scores in prone and kneeling are in the high 90's but he struggles a little in offhand. I was looking into trying to find a LT barrel and where we could buy one and who could change it??"
Assuming you are in the US, would it not be easier to get a US made barrel fitted, rather than a Walther? You could get a slimmer contour, or a shorter length to reduce the weight.
Alternatively a gunsmith could remove the current barrel, shorten it at the breech end and re-chamber.
A lot of top level US BR shooters are using reverse taper bbls. Seems like these could be ideal for position shooters. You can go to the killough site and check out the available contours of the Benchmark bbls. I am sure that Dan would happily send you the weight of the blanks.
15.5 seems like a lot unless you have already added a bunch of wt? We have the light and are just over 14# (legal for a female shooter is 14.3 ish) with significant wt added
There's no wait added to the rifle other than the walther bloop tube. I have enquired on what a new barreled action would cost and if its even available.