I had found out if you have 2 costume cores on same hero (basic one and one new) you can simply use your main costume core for classic DPS and the old one have enchanted with core 25% hp/spirit on medkit and 2s invul/3s invis on medkit. And with simple script you can make:
1. move mouse to location where your other costume is in inventory
2. open inventory(press I)
3. leftclick for equip medkit costume.
4. close inventory
5. use medkit ( i have it on H on action bar)
6. open inventory (I again)
7. left click for equip DPS costume
8. close inventory(press I).
You may ask why would you close inventory in step 4 and open again in step 6.
Answer is because i had found out if i click on item it tends to freeze my inventory for like 0.5s and i belive its faster to close and open inventory than wait for unfreeze + if you make no delay between keys you wont even see the inventory open in your gaming screen.
I use logitech g15 keyboard so i use their macro program.