I might switch to Mata, he's a FK taker. Doesn't play every game as he rotates with Cazorla so probably Petersen as 2nd choice and will take them when Cazorla is playing.
Only thing is.. Mata's shooting and crossing are about 10 less than Petersen's, hence why I choose Petersen for FKs. You say specialists have +15 bonus but Petersen is also a corner taker so his crossing also gets a boost