An an example Juniper MX80 has ingress->egress delay of about 8us, on low-latency cut-through switch it can be <1us (maybe 0.7us). (Remember that cut-through switch can't do cut-through 100% of time, only when egress port happens to be idle!)
1km in fibre is about 5us latency (again, single direction).
Serialization delay @10G for minimum sized payload (46B) is about 67ns (0.067us), by increasing link-speed, you decrease serialization delay.
IP header is 20B, UDP header is 8B, you data is 8B, so you have just 36B of data, which means your ethernet payload will include 10B of trash you MUST send, i.e. if you have something to add to your payload add it, it has 0 latency cost.
I hope you can extrapolate RTT from these, by multiplying the device delay by device count and adding 5us for every kilometer of fibre and then multiply that by 2.
I can't resist adding some thoughts aboutg HFT.
According to this HFT volume halved between 2009 and 2012. Suggesting that the easy wins are gone. I'd love to see some scientific paper or just real data about HFT latency and its affect on profit. I suspect latency which affects trade profits are on another magnitude than latency we're talking about right now. Friend of mine who builds network for one of the largest exchange seems to think it's just customer who do 'lower == better' without understanding the scales.
I can completely understand how HFT was useful when few people were doing it, when you could observe marketA not seeing change marketB does see and capitalize on it. Some are talking about using regulation to stop HFT by taxing each trade making it expensive for everyone, I don't think it'll be needed, I think the window of opportunity is already closing.