Tom and Martin have made their name working on existing courses. I was a member at Tom's Heythrop Park in Oxfordshire for a couple of years, and, given the constraints of the site (one of the most important historic parks in England; permission to build golf was only granted because it was so degraded, and the golf enabled the restoration of the park) I thought he did a very good job. But the constraints were such that the golf was inevitably compromised: it was a very long walk, and the eighteenth hole, which played straight up the Grand Avenue in front of the house was not permitted any artificial features at all, and thus, although a fantastic spot to play golf, was in pure golfing terms rather dull.
I haven't seen Skibo Castle but am told by some good judges that it is really rather splendid, especially since Tom went back and redid it.
IMO Martin's best work is the Ailsa course at Turnberry. Turnberry always attracted a lot of votes in course rankings because it is so picturesque, but given that the body of the course was heavily reconstructed post WW2, presumably on the cheap, the details of the course were not the best imo. It is much better post the Ebert work. I think the famous run down the coast is significantly improved: the ninth works far better as a long par three than it did as a four, pushing the green at the tenth back to the water has created an excellent par five, and the new eleventh (which was originally a collaborative idea by Martin and George Brown, the longtime and late Turnberry courses manager; I have gone on record several times as saying the hole should, as it is completely new, be renamed after George) is outstanding. But there are a lot of less obvious improvements too: the par three fifteenth is a hole I have always liked, but the green used to have a bowled quality that meant the pin could not be located close to the big drop on the right side: that has been fixed.
There is no doubt that Tom and Martin are both very talented architects. Are they so much more talented than their British peers that their current pre-eminence is justified? People will have their own opinion. My only comment is that their dominance of the UK market in recent years has left a lot of courses, especially our links, with similar features. I remember the first time I saw Ebert sand scrapes (I think at Prince's, but I can't remember for sure): I really liked them. But when so many of our classic links have the same features, it starts to get a bit old.