Just how slow growing is the Pyracantha?

Pyracanthas aren't that slow and will tolerate many sites - they're tough !

Faster growers would be Griselinia (cream and green variegated), Aucuba (yellowy green), Eleagnus (goldy aswell as green ones) or Escallonia, those are just a few.

They're all evergreen and the Escallonia's the only one with pretty flowers so to jazz-up the others you could always grow a Clematis next to them so that they grew through the shrub - giving it a pretty flower on it !

There are a lot of choices open to you :)

It can be purchased as a large plant and should suit your requirements. Soleil D'or (Golden Sun) Early summer berries half inch. 10ft. high, 8 ft. spread. sold in 25cm. pots 120/150cms. high. roughly £8 each plus vat.

Pyracantha are full of thorns and are difficult to prune when large without getting very scratched, or children can be very scratched.
Have you thought of Solanu? A semi evergreen climbing shrub very fast grower covered in masses of tiny blue or white all summer very easy to train where you want it to grow. Jasminoides is blue and Album is white. I always kept mine to 6ft high and wide. The house where I grew it still has it 20 years later. I grew (blue) mine where the house met the fence and it was quite shadey

Shade will be an issue with the pyracantha, and the escallonia. have a look at Cornus alba 'elegantissima' - this retains its variegation even in shade. Or even better, what about a Fatsia japonica - likes shade, grows relatively quickly, evergreen, just requires soil that isn't too dry.

I notice that you have not mentioned the impact of a selected plant upon adjacent flower beds or pathways. Pyracantha will suck the ground dry and lift any pathway within a couple of metres. It is often a plant heavily regretted, due to it's eventual size and thorns.
A more gentler solution would be Humulus - the Golden Hop. It is fast - very fast - and is the perfect accompaniament to a variety of Clematii, both of whom attain no real depth to speak of, but can be trained easily in width to cover all you need.
Avoid a Montana clematis with the Golden Hop - each will try to swamp the other.

Pyracantha is a good choice, especially if you want to encourage birds and wildlife to your garden :) It also makes a good "security" boundary,