If you have enough winter chill a modern rose such as 'Sharifa Asma', to become dormant and drop their leaves, thus preventing photosynthesis, I would think that the winter shade shouldn't matter.
Because 'Catherine Mermet' is an Old Garden Tea and can continue to make new growth during the winter, I'd think that the rate of growth during winter would merely become suspended or slowed down depending on how deep the shade is, to make sure I'd ask a local person who has experience growing Tea roses in your area whether the winter shade will make a difference to its' well being.

Best wishes for beautiful roses,
Lux

Thanks Lux,

That's a good point. Roses here rarely go dormant (Radio Times is still in full leaf at the end of winter).
I think i'll try something regardless, i have been impressed with peoples reports on young lucidius.

Thanks again.

Mutabilis is not a rose that would work in your situation, but as an example mine has maybe two to three hours of sun in the winter (which is usually the rainy season so days are not always sunny) and then more sun in the summer and it doesn't seem the least bit fazed by that regimen. Of course my climate is hot and dry in the summer so most roses appreciate shade here, especially in the afternoon.

Ingrid

*Update*

I ultimately went for Marie Lambert aka Ducher because I love the passionfruit scent and the handsome red new growth.

It did defoliate to a mass of twigs over winter, but spring is here and its back with a bang.