Basically, with the health insurers - you pay for what you get. The more you pay Allianz for health cover, the better the dental cover. Few of them cover "cosmetic treatment" in the basic cover - so you need to read the small print to see if they'll do things like enamelling to make a filling/treatment the same colour as the rest of your teeth!
150% reimbursement is not 150% of the cost of the treatment though. It's 150% of the tiny amount that the social pays you, which is not a lot!
I have dental cover upto 150% with my mutuelle. Basically that means I get more back than I would if I had taken just standard cover. I also have an advance payment card which means the part that I have to pay is debited 30 days after the treatment is paid for.
If you have a mutuelle, you should have some dental cover and may, or may not, be able to increase it if you are expecting the cost to be high...