You have to realize, some shops and even some CS reps from the brands themselves, just don't care. Make the sale and get you on your way or crank through the customer concerns respectively.
Burton's own spec sheet on their own site says the Custom 156 is maxed out at 175lbs. You should mention that to the rep when you speak to them next. I'm also guessing the shop employee who put you on that board sized you up based on how far the board reaches up your neck/chin/nose. Bad practice and I wish it would just die out.
Anyway, look into the board I recommended. Arbor also makes bamboo cores. You can go 156 with those due to the durability.
If you wait til next season, you should look into the Never Summer Proto CT. You could do the 157. Very durable boards with their Carbonium topsheet. You also get a 3 year warranty with NS boards.