The positioning of the device is MUCH more important, as it needs to be positioned relevant to the airflow and in a position where it will do good, often this is not the case with DIY jobies though.
The MX5 channels much more air under the car than the F does, but this is not a noticeable due to the vact that the car is front engined. The need to slap metal sheets under the car would not take my breath away, but im not sure what they look like underneath.
The issue with the F & TF is the lack of downforce at the front end to counteract the lift produced at higher speeds, due to the pressure difference the radiator sector of the bumper creates, its worse on the F by a mile.
The TF and F in my opinion would benefit from a properly adjusted spoiler rather than a dedicated diffuser, id imagine to be of any use on the TF it would need to extend well beyong the end of the car (think race S2000's)and therefore would not be road legal