The longer boom will have some additional gain, though possibly not as much as you may expect.
You will have a tighter, more narrow pattern. This will give you the impression of even greater gain.
Now what you will most likely experience is, stations you heard off the sides with the shorter 5 element will most likely fall off the ends of the Earth.
So the group that you "roundtable" with is going to falter. Stations you heard off the corners and sides will diminish considerably. You'll have more rotor usage, as you try to find the "sweet" spot between stations you could split between before and hear both or several before.
This will most likely disappear.
The beam if in a Vertical configuration, will turn into a "BRICK" wall off the sides.
So more rotor usage to find stations others are hearing, that are now blocked by the new pattern.
I also found with a long boom 6 element from a short boom 5, is the stations 90-100 miles out are almost gone unless your pointed damn near directly on them.
The long distance range(500-1000+ miles) when band is open with a Horizontal configuration, that "Brick wall" will diminish a little. However the rotor usage still increases dramatically from before. I have actually missed stations, I could work before on the short 5 that I can't find on the long boom 6.
This is due to trying to find them with the tighter pattern, the difference in receive angle and by the time you find what you believe is the correct heading, the path has changed or gone.
Well good luck
All the Best
Gary
PS: Due to what I have experienced as above, when my second tower goes up hopefully before summer is done. The 5 element short boom will be placed on it, at a different height of 10-15ft higher than the long boom to regain some of what was lost with the longer boom. This should help also when the receive angle changes on the longer paths.