Man if you love the car just go FI on it! You know what engine is in it, you have done your own maintenance on it and you want to make more power. It makes sense to go FI. It will give you close to the power of the m3 without the insurance and road tax of an m3!
I have the ESS unit but I put it on an old 325 running on the wrong tune etc and as a result blew the engine but I am currenntly spending Shed loads of money putting a 3.0L engine in and transferring my ESS unit over to it. Financially it would make much more sense to go out and buy an m3 but if you are already heavily modified or have an unexplainable love for your current car just go do it!!
The torque and low rpm boost of an ESS unit has made me take out a loan to get it back up and running, thats how much love I have for ESS! haha