Yep, I had the same feeling with the 85 wide slalom boards I was on.
When I was fully powered up, I was going like a rocket. Super boards.
On slightly downwind courses, they are truly amazing.
But with very marginal wind, and typically in the lulls, I was all the time trying hard to point the nose downwind to maintain planing on these boards. And often as a result not able to return to my starting point.
This is exactly what the US does very well, i.e. while planing, it points better. Of course it is better in pointing. After all, it has a width of 93 and a standard 58 fin which gives loads of lift. And then the nice thing of it all, is that you're not easily overpowered on it like on a Formula. The less extreme outside position of the foot straps with a more natural upright position and the tail that narrows down like a slalom board make sure of that. Also jibing is really fun.
For those of you who have not yet tested the US against a 85-wide slalom board, please do so.
You'll know what I'm talking about.
And then last but not least, why would you even bother about a 7.8 in 20 knots ? Then you're already a long time on you're IS110 not ? I often start off with my 8.8 sail on the US and when it gets too tough, i just swap the board for the IS122(IS110). That works wonderfully.
IS87 / IS101 / IS122 / US147