seja... I was thinking Allwave, then saw that you were as well.... good choice.
At 160 and 28 years old, you should be fine on the 8'11. It may be a bit of an adjustment coming down from 10' boards, but you are correct... the bigger boards are a lot harder to maneuver on a wave.
I am 220 lbs and the 8'10 Allwave provided plenty of float and stability for me... even in full winter gear.
I paddle flat-water whenever there is no surf. I paddle surf SUP's only.
(I won't buy a dedicated flat-water board, because, if I spend money on a board... it
has to fit into my surf quiver)
I paddled my 8'10 Allwave 5 or 6 miles every flat-water trip... many times a week. It was fine. I don't go fast... but I go. And I have a blast.
You can probably find deals on last years model (new or used)... they are good boards if you want to go that route.