Been boarding about as long as you have, so don't take anything I say as expert advice. However, 2 of the 3 boards I ride are technically pretty undersized for me at my weight! I ended the season at about 235-240lbs. buck nekkid! (...I apologize for that mental image!)
So geared up I was likely in the 245-255 range riding a 159 & 157cm board. (The 157 is a blunted tip NS proto CT which is supposed to ride same as a 159.)
You could probably expect a too short deck to feel softer & flex more than one in the correct range. It would certainly feel softer relatively than the mfr's stated flex rating. Which could help to make butters and presses, easier or
more difficult depending!
Also, I probably wouldn't be using a vastly undersized board for trying to get any big air! I read about guys busting appropriately
sized boards hitting Big Kickers somewhat regularly on this forum. So I would think going too small and trying big jumps wouldn't be a smart idea! (...for your safety or the boards!)
Aside from that, and maybe losing some ability to float if it gets really deep? I didn't find too much in the way of disadvantages in so far as just cruzing & bombing the groomers! If the board was really soft to begin with, it might flex too much and wash out as you chatter thru any really hard/tight turns at speed!
Ultimately, unless you have a very specific reason for, and can identify the riding/terrain handling advantages you're hoping to get by going a lot shorter? You are generally better off staying at least close to being within the weight range for your deck. But unless you are so big you will snap it in two just by strapping in? I'm not sure it's worth worrying about too much!
I'll be curious to read any additional replies to your question! Especially any in rebuttal!