Impatiens prefer shade, but if they have to have half sun,they prefer it to be morning sun,and afternoon shade,as the afternoon sun is more scorching.....
Having said that,I have some impatiens that receive afternoon sun,and they don't look as good as the full shade ones,but for me,they look better in the winter.
It's up to you...But if it's for a rose bed, look for sun lover plants,like petunias,or daisies.

The only secret to keep impatiens in full sun is regular watering, sometimes 2 times a day, they won´t grow as big or bloom as much as being in partial shade but they bloom enough to make the effort well worth, and in your zone they are perennial.