there is no mastery crystal (edit: i am wrong). i think mastery is better than crit once you get to 2000 crit or something... for pve

these are the numbers i get for simple dps model assuming 110 accuracy and 703 alacrity

base crit mas surge

1500 2124 7720 0

2000 2250 7594 0

2500 2364 7480 0

1500 2184 7660 0.1

2000 2307 7537 0.1

2500 2418 7426 0.1

for 6.0 at 1.3 gcd and 110 acc, i am seeing a 9 dps decrease if using as power crystal instead of crit (18 if using two power crystals)... this is at 1415 crit and 3206 alacrity

for 1214 alacrity we get 3407 crit. now when exchanging 82 crit for 82 power, it is only a 0.55 dps loss for using power crystals.

this is for an average hit of 2546.... you can see similar discussion here:

https://www.swtor.com/community/showthread.php?t=965720