Personally I am very into the effect of weight but I personally can't see the point of taking a heavy car and trying to make it lighter to improve lap time on a track day. As I said earlier you will greatly improve your lap time and enjoyment with some decent instruction and it will cost a fraction of trying to lighten the car.
Also Zag's point is a good example of many a true word is spoken in jest. I remember standing on the pit wall at Silverstone waiting for our Free Practice to start watching young drivers in a Fiesta championship (we were running a Formula Ford) a father was asking the guys running his (16 year olds approx) son's car that was the slowest in the field by some margin what they could do to make it quicker, intimating money wasn't an object. When their session ended the cars came in and this dad went over to his son's car and when this lad got out he must have been 18 or 20 stone so he probably weighed more than double all the other kids his age he was racing against! Need I say more?
If I want a light track day car I would start with a light car ie my Lotus 340R or similar.
Having said that if you are going to remove heavy items from something like a 4200 you also need to consider how it will affect the weight distribution because if you change that it can alter the handling characteristics of the car.
For example when we ran the Ginetta G50 in the 09 FIA GT4 Euro championship the BoP got really silly and the extra weight imposed on us had to go in the FIA approved ballast tray which bolted into the area where a passenger seat would be in a road car and this played havoc with the balance of the car. To compensate Ginetta produced an extra thick steel rear floor to replace the GRP one to help us add weight legally elsewhere in the car.
Even on a road car you will be amazed at the difference doing the corner weights can make to the feel and balance of the car. We did the corner weights on my old Impreza STi Spec D and it transformed the handling, made it much more like my previous WR1 in terms of handling balance.
As an aside I am lucky enough to have spent a fair amount of time sat in on instruction with one of the worlds best race driver coaches, this is obviously way beyond track day stuff but alot of what is being taught is how to manage the weight transfer in the car as you drive round the circuit and the driving techniques to best achieve this, fascinating stuff.