…will it have on my bike setup, and is it the right thing to do to lengthen my TT a bit if I don’t want to change my stem length?

Can only think it might effect KOPS and weight over the front wheel?

Wouldn’t try and an analyse it too much, just buy one and try it as, unless you run your current inline seatpost at the very back of the saddle rails, you can just move the saddle forward again negating any effect of the layback anyway.

Plus as soon as you’re out the saddle it doesn’t matter anyway. A layback post might put your weight further back over the rear wheel although that’s only going to be noticeable on steep climbs but then just perch on the front of the saddle

With shorter legs and a long body I’ve pretty much always used layback posts, only issue now is there’s less choice when it comes to dropper posts

Can only think it might effect KOPS and weight over the front wheel?
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KOPS? Knee over pedal spindle? If so, then yes, having your saddle too far back or forward can mean knee pain or not.

Right. But ok for lengthening my cockpit a bit then.

Give it a go. Maybe buy a cheapy first to see if it suits.

Thats exactly what I’m doing. I’m in between frame sizes, went small feel OK but people comment on my pics saying it looks too small. I’ve slammed the stem, this’ll be my last option to get more length across the TT.

Slamming the stem won’t effect the length of the bike in any meaningful way…

No, gets me lower though. Sorry that comment was’t in relation to lengthening the effective TT…