From what I know a hasted/Speed enhancement cannot go beyond the highest level effect and do not stack. So if someone was sped and haste was cast upon them then they get the benefit of the higher effect. In the case of Multi strike weapons I believe that this is part of the weapon as an enhancement so the gain would probably be net 1 extra attack beyond the haste of speed version spell.
Please let me know If I am incorrect on this matter as this is how I would rule.