It's horses for courses.
Why do you want a 500?
I bought mine because of the tuning potential, what I wanted to tune it to and for the heritage and prestige of owning what is considered by many as being "THE" Cosworth.
I personally would never want a normal 3DR, nor a Saph..... but thats my choice and we all like things for certain reasons.
I have never driven a 3DR on a T3, so cannot comment, but have owned RST's and Escort Cossies, so appreciate the differences in driveability between different types.
I've had many mods done to my 500. While the car is still laggy when being driven normally, if you change your style to selecting the correct gear for the T4 to ensure you are on boost or you drive it properly through the gears, it is awesomely quick (have you tried driving a T4 tuned motor past 7k in 4th & 5th!!... the road somewhat blurs). It is certainly quick enough for me and I think I will stick at the power level I am at now.
I personally dont care how quick or slow it is against others, that is not my motive. I get more self satisfaction when people admire it when it is on the road or parked up..... and in that scenario it makes no difference whether it has 20bhp or 500bhp, or has more or less lag than a T3....... apart from when the bonnet is open and people start pointing at the nice bits that make the power!