I'm also the one who is putting points into rockets after turrets, the pushing is usually very good then.
the biggest problem for heimer i find mana and health. I have mana regen quints and seals/lvl, abilitu power seals and magic pen marks. with utility mastery clarity and teleport usually no big mana problems, but i find heimer most effective when he has mana without the end.. to get that i need chalice and archangels, and then i can spam spells without end, and if mana is kinda low, then pop clarity and done again.
just in this case i have totally no survivability and rely 100% on teammates. i started now going rod of ages instead of archangels and it does increase survivability, but gives less AP. however in some situations i run out of mana if the push is very long.
since its always like "we wont start the fight till we bring heimer turrets down", and then you need enough mana to constantly rebuild them, keep throwing nades to towers, and firing rockets to champions when possible.