If you're targeting 1 pound of P per thousand square feet (about what bag rate will do, give or take a fair bit), you'll get an insignificant difference in N and K.

You shouldn't notice a difference.

That having been said, you may or may not actually need the P. It's often helpful when seeding, however, as the young roots aren't as effective at absorbing nutrients from the soil and don't have the surface area that adult root systems do. A temporary excess isn't a problem on P, either (although it's an iron antagonist and might temporarily render the young lawn a lighter green than it would have been).

Overall, if it had been $3 a bag cheaper it would have been worth it. A difference of $17 per bag is a no brainer!

That's what I figured thanks. I'll probably go but up the rest of their supply today because I'm helping my friend on his yard in a few weeks.

I don't have a control to test against but I will say that using starter fertilizer in conjunction with proper seeding prep worked amazingly well for me... Compared to all the "how to's" for seeding mine are doing exceptional...

My seeds have been down for 6 weeks and I they no kidding were growing almost 3 inch per week in most places, and I had to start mowing 2x a week at three weeks.... Don't know if the P really helped but the whole process sure worked great. My yard went from centipede ugliness to pretty level low cut bermuda in a matter of weeks. It impressed me

Great!

Major kicks in health when you use starter might be an indication of low phosphorus levels in the soil. A soil test might not be a bad idea, from Logan Labs.

Over time, insufficient P in the soil will cost you grass health, and possibly its life if it's deficient.

Mine was dreadful when I started out. Nowadays it's...well, a little high, actually, and really high in the gardens.