Not specifically, but it will certainly roll-off some treble. If you are likely to use it with the tone pot rolled back a fair way, then it won't make much difference anyway. 250k will move the pickup's resonant peak back a bit, so you lose some treble but gain some upper mids.

The Alnico II magnets in the Rebel 90 provide more clarity than the more normal Alnico V normally used, but being a weaker magnet. provide less output. This is no doubt compensated for by the extra windings.

On my ES-1/330 style, my Creamery P90s are Alnico II and I really like the sound. I used a 500k volume and a 250k tone pot on them. But then I've also got some IronGear P90s with AlnicoV and a slightly higher quoted DCR than the Rebels and they are pretty similar with lots of detail and clarity.

My own opinion is that you loose a bit too much detail in the sound by going to a 250k volume pot with a P90, but YMMV.

Also, don't forget that the rebel P90s have nickel-silver covers, which themselves will reduce the treble a bit through added capacitance and eddy-current effects, so won't be quite as bright as P90s with plastic covers.