What these guys have told you is true. If you do a PI head swap, you might as well get the heads ported, polished, have the valves oversized, get ss valves, and put a high lift cam in there like the VT engine state 2 wild N/a cam. Combine that with a good intake setup like either the new Fox Lake intake or just go with the Accufab upper intake over the PI lower intake, you'll definately see a good jump in power. Combine all of this with a good tune, some exhuast work, and maybe a set of u/d pullies, you can pull over 300rwhp. However, 350rwhp is another thing. I dont think even with all the above you're going to put down that kinda power. Now, if you throw in a stroker kit, then yeah, you should hit 350rwhp but thats quite expensive.
As stated above, boost is the best way to go to make raw power. However, if you run boost, you run the risk of blowing your pistons and throwing a rod. The bottom end of these Modular motors aren't meant for boost stock from the factory. If you plan on running boost(more than 6-8psi intercooled) you had better plan on upgrading the lower portion of the motor to forged pistons and forged metallic rods for adde d strength. The crank will be fine as its an Iron crank.
Either way you go, whether its naturally aspirated or boost, you're going to be investing some serious money. The choice is yours. Just be sure to investigate the pros and cons of each setup before you make a decision. If you have any other questions, feel free to ask. Be sure to also use the search feature of this site to find other articles written about such things as superchargers, n/a setups, and nitrous.