There are show cats and then, there are SHOW cats.
What your breeder probably meant is that she is show-able, and not of pet quality. But in a cat show it is always the TOP 10 cats that will final.
Let me put it this way - there are Champions, Grand Champions and then, there are the Best of the Best.
An easy example would be like this - look at my last litter...
All 3 cats are of show quality. At an early age both Tom and Tink proved to be the better of the 3 cats - as proven in shows. Spot had the problem with her Spot. As of right now, and I hope I am correct, Spot and Tom are Grand-able as Grand Champions (entires) but Tink has developed a slight jaw problem that has made me think that she is only showable as a Premiership cat and yes, she will Grand there too.
However, none of the 3 IMO are of Division Winner quality (ie. Top show like my Bulan or my Suria). A top show cat is one that finals at a minimum 6 out of 8 rings or in a 4 ring show, 4 out of 4 rings.
Be not dismayed - that is the nature of cat shows. As a breeder, I am frightfully honest with my new show homes. I will tell the new owners that the cat is Grand-able but it may take some time or I will tell them that the cat should be granded in no more than 3 shows.
If your breeder has said that she IS showable, then trust him on that. It means that the possibility of at least 1 final is there. But in the end, to me, even a Breed Win over a large breed class is pretty damned good!