I came up with this archetype after noticing that a Synthesist cant use his summons while in his Eidolon form. This bumps up the Eidolon to a full 20 HD creature over 20 levels instead of just 15 but you give up your summon monster ability all together. I think its a large enough hit to equal the extra power. And you can take this archetype along with Synthesist for extra effect. Below is the link.
I am not that fond of it.
It seem a bit unbalanced the it gives up a secondary ability to grant a significant power boost to its primary ability.
For the Synthesist I really don't think he need it. Making him effectively a full BAB class, and granting all his bonusses earlier, while he is still a spell caster, does seem a bit unbalanced toward martial classes.
If it were me, I wouldn't let it stack with Synthesist. They both change the Eidolon class ability, so they really shouldn't.
While it is a boost in power to a primary ability its by no means giving up a secondary ability. The summons class feature could be considered "the" primary class feature. This combined with the fact that the Synthesist really does give up alot by combining with the Eidolon to begin with. If you think about a base Sythesist negativly effects the summoners action economy with very little gain.
This archetype gives up a big boon for a small bonus. But a bonus that really helps what a Synthesist wants to do... which is be a melee powerhouse.
The summon monster ability can be considered a primary ability, but that depends on the build. Those I've seen has all been about the Eidolon, and the summons has only been used as a backup.
My concern regarding the synthesist is compared to martial classes. It is already a strong class, and with this archetype it is going to outshine the fighter severely.
He is going to have fewer feats than a fighter, but they aren't as important since he can focus on his bunch of natural attacks. For special abilities, his evolutions is going to make up for the lack of feats. On top of this he gain access to abilites such as pounce that the fighter can't.
He got an extra d8 hit points per level. His is going to have high mental stats because he can ignore the physical ones.
He is getting natural armor, possibly armor, and shield bonus to AC, which is lower than a fighter a low levels, but is going to outshine the fighter at mid to high levels AC-wise.
The synthesist is of course able to cast a long range of buff spells on himself as well.
The fact that one might argue that a normal summoner is better than the synthesist does help this issue. For a normal summoner, I would rather avoid the discussion on whether or not the eidolon is stronger than a fighter. This archetype would IMO move the balance in the wrong direction.
This is completly overpowered.
Synthesist is already considered overpowered or borderline overpowered. Giving up something thats highly circumstantial and rarely used in a synthesist for a major boost in its primary ability is completely broken.
Its like an archetype that says a fighter cant use swords or picks but get +4 attack bonus and damage with axes. Sure, you could build a fighter around swords and then its bad, but if youre going for axes its a huge boon at no cost.