There are studies showing, for example, that 145s are more efficient than 175s but confirming such findings in your own situation is difficult. You can experiment with inexpensive crank shorteners but they change the Q-factor and even if you were to bite the bullet and purchase new cranks, you'd need to raise your saddle by more than an inch which will put you into an more aggressive riding position if your current setup prevents you from easily raising and possibly bringing back your bars. Going with shorter cranks makes it a lot easier to pick up your pace.