I can give you a definite answer as I contacted TG ROP service in Hong Kong to ask about baggage policy for sports equipment (golf/ski luggage / tennis racquets).
GOLF/SKI LUGGAGE:
"We regret that golf/ski luggage is part of your total allowance of the checked baggage. For ROP Gold member, if you travel on THAI or Star Alliance carriers, you are entitled 20kg in additional to class of checked baggage entitlement."
TENNIS RACQUETS:
"Due tennis racquet is over-sized than the allowance of cabin baggage, it is required to checked as a checkin baggage."
I just checked the lenght of my tennis racquet -67cm- which is rightly said by TG customer service "oversized" (bigger than 56x45x25cm)...
Apart from that, I found some infos on TG UK homepage regarding sport equipment allowance, which might be helpful for you as well:
http://www.thaiairways.co.uk/pages/f...ai/luggage.htm
To wrap it up:
Your allowance on first class is 40kg incl. golf bag, on business class 30kg incl. golf bag -regardless whether it is domestic or international flight-.
If you are a *A gold card holder, you get +20kg on top.
But TG will not trouble you when leaving from USM, if your luggage is slightly over the allowance, in particular as you are booked in on first class for your onward journey. Contrary to LH....
Enjoy playing golf in Samui!
Cheers ^