I've done very well with a shotgun vs AEG's. BUT.....I've also had my ass handed to me when I've gone up against a mostly upgraded team and I'm with a stock gun.
You can go against upgraded guns but you have to have the right field to do it. If you don't have enough cover....your toast. It also helps when you've a decent team or crew to back you up.
My AEG should be running ( havn't chrono'd ) around 360-380. I like having more range. I am tired of being outranged by 100 feet....but I'm also aware I'm going to be stuffing more money into keeping the gun going aswell.
In short, you don't need to upgrade. But your feeling of your "need" to upgrade will be affected by how you play both by yourself and your team and more importantly by your field(s) that you play on. Playing stock has it's own rewards.