If you can tune the 135i, then you're allowed to tune the Lotus. Running a smaller pulley, a Charliex tune, and a cup GT3 airbox on an Exige = kills most things.
I would say Lotii, Caterhams, Atoms, and Nobles are all in the category of lightweight, street legal racecars. There are benefits to having a heavier car, but track times are not one of them. Arguing that racecars are not faster than luxury sports cars is like arguing the evidence for creationism. Look at stock for stock track times for a Lotus Exige, and then for a 135i.
If you want cup holders, a good sound system, sound deadening material, a sturdy and safe chassis (although the Lotus tub is nice for weighing 160 lbs), a cushy suspension, and seating capacity, your car will need all the HP in the world to go faster than a proper track car in a straight line. If you have a heavier car, it will never, ever corner as well. That is the sad reality.
Why do you think so many people autox miatas instead of tuned escalades?