It seems to me that if your Rampage isnt doing the same or more damage than Asuran Fists... then you are not setting up your gear properly. FYI decimation sucks! Rampage onry. Rampage if you have the proper ACC gear and balance of STR and attack then you should absolutely have no issues breaking into that desired damage range for Rampage. As for Simian Thrash there is a few things to consider. Being a full time MNK from FFXI for over 7 years i understand H2H. The one thing to remember with Simian is ACC ACC ACC and more ACC. Attack and STR or whatever modifier the WS uses take a side seat. Up the ACC and ACC modifier equipment, hopefully still DEX, although that is still an unknown factor and you should see your damage increase.
On many occasions my Asuran fist would do more than rampage and many other melee single handed WS. The WS i couldnt keep up with would have been Great Axe and Scythe and of course RNGs who just pwn everyone gotta love us lol