2452H-SL in a 2-way

[Zilch]

I see a start attenuating around already at 1,5kHZ. Is it the horn or the driver?

It's the combination, and you'll have to measure yours. You need at least minimal measurement capability like RTA to do this stuff. SPL meter and signal generator, maybe.

It is perhaps better to use the 2435HPL?

Not with respect to lower cutoff, I wouldn't think. 2452H-SL is a 4" large format driver, and 2435HPL 3" medium format. I've probably posted measurements of both using 2352 horns here somewhere.... :dont-know