When I play IMP at lower level, 6-8, the single most important defense component was the SC.
And the most common rebel attack was heavy+ medic combo.
The base design for IMP should be to defend that combo. This mean:
- Put snipers and bikes (L8 or higher) in SC. 2 snipers + 3 bikes (32 UC) was my favourite SC combo at L8.
- Place SC in the middle of base, walled in, with shields covering. Check the SC triggering radius to make sure that it can't be easily triggered. Make sure no drop zone (empty spots) within the base. I saw many in your base.
Most turrets are not very effective against heavies medics, as the rebel medics are quite strong. So, I found using concentrated base design with turrets closely placed together was effective for me (yes, I know concentrated turret design is prone to the b-wings spam but at least it can do damage before the medics would heal heavies right back). Watch replays for common attack start points and put tons of traps there.
Take care of your SC and it will take care of you. Snipers and bikes are your best friends. Turrets are for decoration