How is contributing to combat stats not improving combat performance?
Gear on Thrawn directly affects the stats you are saying are lower, so extra pieces your opponent has is adding to the speed, potency and damage. Compare the numbers in the blue brackets on the stats screen to see how both players Thrawn's are contributing to the stats.
Stars do boost combat performance, as they increase the STR, TAC, AGI and Crew multipliers which add more stats.
If you want more informed help from everyone, put up screen shots of both Thrawn's and both Chimaera's stats, that's what @Darthpedro was after.