Elliptic curve commitments mod p As far as I understand secp256k1 is defined over the group p with 

p = FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFE FFFFFC2F

I don't really understand how out of bounds values are handled in particular with homomorphism of the commitments.
Assume I commit to the value 5 which would be $G + G + G + G + G$ and then commit to the value p + 5 which would be $(p+5)G$ will those be the same commitment?

Based on this assumption I have implemented the following javascript code (using elliptic library ):

it('Test mult property of commitments', () =&gt; {
    const T1 = ec.g.mul(secp256k1.p + 5n);
    const T2 = ec.g.mul(Maths.mod(secp256k1.p + 5n, secp256k1.p));
    const T3 = ec.g.mul(5n);
    assert(T1.eq(T2));
    assert(T2.eq(T3));
});


In this example, T2 and T3 are the same, but T1 is different, so it seems like my assumption is incorrect, does this mean I can commit to values greater than p?
You are confusing the prime $p$ over which the curve is defined (the coordinate of the points are all defined mod $p$) with the prime $q$ which is the curve cardinality and is also prime.

We have $qG = G + \ldots + G = \infty$ the neutral element (like $0$ with the addition with numbers).

Taking your example, you have: $T_1 = (p+5)G$ and $T_2 = 5G$. Of course it is not the same since $p + 5 \not\equiv 5 \bmod q$.

You will find the value $q$ somewhere in the parameters of the curve and if you replace $p$ with $q$, the equality will hold.