I can kinda help however it's not a direct comparison of the 7'6 Speeed but rather the 7'8.
Over the past 3 years, I've tried all the Speeed's ranging from 7'8 to 8'10 & I initially settled on the 8'5 which cemented it's spot in my quiver. I subsequently dropped a few kegs + I now paddle on a regular basis so I traded my 8'5 for an 8'2 approx 1 year ago.
At the start of this year, my objective was to find a low volume 'short' board that I can also travel with (some airlines get grumpy with anything >8') so I tried the 7'8 & 7'11 Speeed along with a 7'10 Acid & the 7'8 Speeed came out on top mainly because the Acid is too much like hard work + I already own an 8'1 Acid which I love. A few things to note:
- I'm 80kg / just over 6' & I can only use the 7'8 when it's clean otherwise stability is an issue i.e. there's no way I could handle the 7'6
- it sits a lot lower in the water vs my 8'2 Speeed so I definitely sacrifice some glide into waves
- that trade-off is well worth it once I'm on the wave because it's fast / manoeuvrable & feels alive
- it wouldn't be possible for me to paddle a traditional shaped board @ 25.5" e.g. my 8'1 Acid is 27.25" wide
- I rate the 7'8 Speeed as my most playful/fun board although, I use my 8'1 Acid more frequently because I find it fractionally more stable (despite the pulled in nose & tail) @ 89L + it's got that X-factor...for me
Bottom line, it all depends on what your objectives are & like Traff mentions, how much you want to push things (age / weight / your local conditions & skill level all count) but if you want a versatile & fun board, the sub 8' Speeed's are a blast

PS - what really sold me on the idea of a owning a low volume Speeed was watching guys like Marty & Casso tearing it up on 1!