Hey thanks for answering my questions ... So it isn't worth upgrading it to IS/JB as the system isn't good enough to handle them and the battery life decreases?
And also have you used any of the custom GB roms? And do they effect battery and lagging on the phone?
Thanks
Like i said, there are advantages and there are disadvantages, its all upto you, if you can live with the shortfalls then go for it otherwise stick with GB.
ICS/JB are very resource hungry systems, they are quite high on graphics as well, so in short they take up a lot of ram and hence the battery.
I have used several GB roms, but mostly cyanogenmod based, i don't prefer stock based that much. Battery and lags are a simple compromise, if you want high performance and looks(graphics) then you will have to make do with decreased battery, custom roms are built to give the user something better than what the stock samsung is giving, that includes better performance, better looks and better features.
You will never find a custom rom that gives a better battery life than stock, but you will also never find a custom rom that is not better than stock.
If used wisely then you can easily get about 15-20 hours out of a custom GB rom.
In short, custom is always better than stock, but choosing between 2.3 and 4.0 is up to you.