Well I'd strongly recommend an upgrade, as Shaders are standard today.
On the other hand (as I have seen the gameplay video on Youtube): Use as highly detailed textures as possible, turn on anti-aliasing, use higher-poly models with - again - better textures. Improve some (or most) of the movement code (syncing animation and movement, no "strafing" cars, etc., etc.) Add effects (e.g. a proper explosion for the hand grenade)
This would polish things up a lot, I think.

Edit: I forgot: Probably better get rid of all the real brands in the game and make up your own (as they did in the real GTA series). I think Subway and Audi (among others) are not amused when their (trademarked) logos and designs are used in an indie video game portraying violence.

Another edit: Shaders won't make your game suddenly look great. Professional games do look good because of passion for detail and that stuff I mentioned above.