I'm fairly happy with the multikill as it is, I haven't had the chance to get a triple kill yet, so it's just 1 extra point every once in a while, not such a noticeable difference that I think it needs changing, but a nice addition anyway
In terms of the spawn timer, I think I'd like it more at about 25 points rather than 30, people seem to buy witches and other such upgrades first rather than this, and it'd make it that bit more appealing to buy, but I can understand if it's left as it is because I haven't seen the full use of it yet in a full game