I have 2 hunters that have shivers and it doesn't affect their performance one little bit. One of them struggles to walk backwards and can be a little tricky on one back leg for the farrier but once he relaxed he is fine to do and the other just tenses up a little when being shod, again once relaxed no problem at all, he's the one in my pic!!
My 17hh IDxTB has shivers, has apparently had them since he was 5 and they've never got any worse (he's 15 this year). I can't feed him anything like competition mix as it seems to make him seize up, so hard feed-wise he just gets chaff and high fibre nuts (with sugar beet if needed) and hay/haylage. He struggles to pick one back leg up, but I think that's more because he can't support his weight on the opposite hind very well. He's ok for the farrier but he's got a good trick of wedging him up against a wall, and horse seems to know he's got to be good for farrier.
Can be a little stiff to work in when schooling but I never know whether that's really down to his shivers or the spavins he has.
This particular horse is on loan - have had him 2 years this August and it hasn't deteriorated - and it wouldn't put me off buying one with it if it was only mild. He hunts, he jumps, he does dressage, he hacks, everything.
Really good question. I know of a mare for sale who is absolutely fabulous (international quality) and she has shivers. I know they still want a ridiculous amount of money for her by our 'normal' standards, but her price tag is a fraction of the price she'd command if she didn't have the problem.
I've had my boy 10 years now and the shivers has only really been visable in the last 2 years- he holds his leg up and out - I probably see this once a day whereas before I saw this once a week.
He is a little awkward with a back leg when its being picked up but is fine once hes found a comfy position. He's still fine with farrier but has always struggled with rein back.
He's 14 this year and has competed dressage (moves like a dream), showing and bsja. Only reason he doesn't compete now is because I have no transport
Its really not a problem but I wasn't aware of it when I brought him but Im glad of this as it may have put me of and he's a superstar!