The factory system should only be several hundred feet without any obstructions if I remember correctly. This means line of sight. I personally was going to buy the OEM version remote start. That was until I then found out I could accomplish the samething with using CompuStar's T-Harness for less money without sacrificing quality. I press the OEM keyfob lock button 3 times and it starts right up. The entire cost plus labor was at least half of what Lexus was going to charge me. Not to mention, if you need more range from the unit, you can add a longer range remote or start your car from a smartphone (Drone).
I am actually from the UK where I do not think the remote start is on our options list so I was looking to buy it from America and have it fitted over here. The system you are talking about sounds good but I am not sure it is available in the UK but thanks for your reply, much appreciated.