To comment anything over vw polo,one should remember that same polo is participating in POLO CUP after undergoing some performance upgrades,and that is done by finest reputed people in this field. IMO
the best Indian cars for modifications are:-
Gypsy:- For its rugged body and easy-to-modify nature of everything it has.
Lancer:- A world class sedan with rally dna inside
Swift:- A worldclass rally design,ready for all types performance modification and a good suspension to handle it.
Polo:- A jewel of german engineering.
Skoda:- A luxury car maker whose vehicles,specially sedans can withstand performance upgrades very well.
I have voted for the above vehicles because they are well tested with good results for performance upgrades with good results thereafter.
Well,its not hard to modify a car for performance,but it is hard to make the vehicle stable all the time to support that modification.So the design,body frame and stability of which ever vehicle qualify this requirement, is the best car for performance mods.