Hi Gencion,
The 1150-L wing is a dedicated glide wing. It's glide is totally remarkable. They were designed as a dock start and pump wing. I use it for Downwinding where its glide is just amazing, I can glide through lulls with a little pumping and overtake waves ahead for the next drop. Is it easy? no, not as easy as the W1250. They are technical to start, especially if you've not used them before. Once you get the hang of it, no wing starts easier or is better suited to what you are doing.
The high aspect but thick profile means it is low in drag, high in glide, it has a high (for its size) top speed. The downside of the Leviathan range, especially in the larger sizes (1350+), is they aren't super agile. The 1150 is agile enough and no problem to turn, but the 1350 takes a more concerted effort to carve around.
I personally love the 1150 and it suits my downwinding in Perth conditions for all but the really windy big conditions. For 25knots+ winds I like the Razors in 980. Great speed, good glide and very agile. Suited to surfing and downwinding. I have used the 820 with kite foiling and it was amazing. Top speed of around 27 knots for the 820.
But in answer to your initial question: Yes, the 1150 will be faster and have better glide and less drag than your 1250. I think it would overlap all three of the wings you currently have (depending on how you ride).
DM