Had a TB mare whose neck was not unlike the neck in the picture - pretty typical TB neck. We roached her mane because she had a draining wound at her withers that was making the mane all icky and gross. She looked fabulous with the roached mane (wish I could find a picture) and while it was growing out - and it grew out beautifully.
I think the matter is probably already decided in this case, but my vote would be to roach now - the thinning makes the current mohawk less than an ideal mohawk.