I am 100% positive that whatever Heather designs and makes for horses has the horses comfort at the VERY top of the list. Lets face it she 'tests' most of her designs on her own horses before they go into production for the general public. Do you think she would put anything on her own horse(s) that could potentially harm them?
Im a recent convert to a treeless (SBS Flexion) and I would NEVER use a treed saddle on any of my horses. My horse went from having a sore back that errupted in scabby sores and bucking into every canter transition not to mention running around the stable whilst I tried to tack him up and flinching terribly at the girth being put round to a relaxed, happy, uphill actioned horse, winning at affilliated dressage (and hes only 5!). With the use of clicker training and his special exercises he no longer runs away from the saddle or the girth but stands with his ears pricked.
The research that went into the SBS and the research that has gone into the Fheonix is second to none.
I was wary of the treeless but if my horse's reaction isnt evidence enough - then I dont know what is.
And of course you can 'trial' a Fheonix to check it out for yourself - not many saddlers would be THAT confident in their saddles to say that.