In terms of pricing, Zen-Fire seems to be the best "retail" solution. But as you said, you need to be faster, so you can try some faster and more serious options:
QuantHouse - CME's Level 1 market data will cost you around 1500 Euros per month. They have points of presence in most local financial centers in Europe (Stockholm, Frankfurt, etc.) so you can colocate near to them, if you wish. They aslo operate their own fiber network, so you will get your stuff from USA very fast.
FixNetix - All the same, actually :) Maybe some little differences in pricing.
Regarding your question about higly correlated instruments - i do not know any, and as i know, S&P500 Index futures (big and e-mini) is traded only on CME and somewhere in Asia (am I wrong?).
You can get free real-time S&P 500 CFD data from a forex broker like FXCM. You can open a demo account and test the quality of the feed through their API. Note that it's a CFD, so there are some small differences to ES. The quality is probably not as good as Zen-Fire.
Another DTN product, Nanex NxCore is a very good choice for a high-quality "retail" feed. It will suffer from the standard latency issues of transporting the feed across the internet, but it is likely the highest quality feed one can get on the retail end of the spectrum.