I'm with gboulton on this one; you need a new LHS...

Thrust angles are important for a number of reasons and can affect flight characteristics. It's my guess that you're not really interested in that right now, nor am I really wanting to try and explain it all right now.

Suffice to say the manufactures have done flight testing on their models and established thrust angles which work best for that particular model and recommended engine range. Rotating the engine does nothing to affect thrust angles as gboulton said since the engine's centerline does not change in relation to the firewall.