Taking level 10 gangs Godfather Five Families

Attacking with just Professionals is not the most efficient.

I still think it's better to attack a level 10 Gang with:
Arsos + Professionals/Snipers + Trucks OR
Just Snipers + Trucks

This is why:

A level 10 Gang does 241280 damage. This damage will be divided by the weakest units you send (with exception of Transport units).

So let's assume you have Medicine and Muscle level 15 and you have level 8 armory Gold set. Then you have 195% bonus on Attack and on Health.
This means:
Each Arsonist has 661 Health
Each Professional has 11148 Health (your ideal)
Each Sniper has 19913 Health

So depending on what you will send, you will lose:

241280/661 = 365 Arsonists OR
241280/11148 = 21 Professionals (as in your ideal) OR
241280/19913 = 12 Snipers

But now comes the training time (the absolute amount depends on your hideouts, but the relative amounts are the same for everyone!):

365 Arsonists x 13 seconds = 79m 5s
21 Professionals x 258 seconds = 90m 18s
12 Snipers x 438 seconds = 87m 36s

Conclussion:
If you have your Professionals taking all the damage, you will need the more time to replace them than with Snipers or Arsonists. This is easily predicted by the fact that Professionals are damage units. Arsonists are Health units and therefore have more health per hour of training. Snipers are somewhere inbetween, because they are Balanced units.
So why not send Bruisers or Tommy Gunners then? They are Health units too, someone might think then. However Level 10 Gangs have Enforcers with 150% bonus damage against your Bruisers and they have Professionals which have bonus against your Tommy Gunners.

So what do we send to a level 10 Gang then?
400 Arsonists (you will lose about 365 depending on your Armory items and Medicine level)
+ 650 Professionals (or Snipers) for the damage
+ 22 Trucks for the load.