No need to go for a full speed pack; that will make no difference at all.
First thing is simply to buy a SIM for 50 baht and to try it out in your apartment with a cheap(perhaps 10 baht) full speed day pack.
Once you have clocked the speed available to you that way, then you buy an unlimited data, capped speed pack to match. If 9 Mbps available, then you buy a 10 Mbps unlimited pack; if 14 Mbps, perhaps a 15 Mbps or 20 Mbps pack. If more than 20 Mbps, you can buy a 30 Mbps pack.
Personally, in Jomtien, I had good results with DTAC, though that was nowhere near Atlantis. Everywhere else, AIS was the best. Better to buy both at 50 baht each and test them at the same time. Then you can add the unlimited data pack to the one that wins.
Also be aware that the DTAC connection can be asymmetrical ; e.g 20 Mbps down/4 Mbps up. AIS tends to be more balanced if you can get a good signal.
Buying a full speed pack won't get you higher speeds than are available from the network at your location; so ignore the suggestion of buying a full speed pack in the hope of getting higher speeds. Full speed pack is only for testing. i.e *104*851*9# for DTAC or *777*70# for AIS will give you a small test pack(100 MB by volume) for 10 baht. Use that to run your speed tests and then come back here to quote the speeds obtained and for the method of adding the appropriate pack.