Boil Volume

For my first brew, i used 1.5 gallons for the boil, then added that to 3.5 gallons of water. I am now reading that the Boil gravity can effect the bittering hops, and ultimately the IBU. Is 1.5 or 2 gallons enough or should i upgrade to a bigger brew kettle?

Makes sense to me. But honestly, if you're making 5gal batches you should move up in pot size a bit. IMHO

From what I understand (limited) it's not good to boil more than 2 pounds of extract per gallon of water. That may lead to what you are hearing about hop utilization. For extract it's o.k. to boil 3 pounds of extract in 1.5 gallons of water... then after flame out add the rest of your extract and stir until it's all dissolved.

I've only done this once and it turned out as good beer. After tinkering with a small pot I ended up borrowing a friend's turkey fryer and pot. He uses it once every 25 years... so I'm safe to boil much larger volumes.