It isn't really that useful to think of the sightreading tests for a graded music exam in comparison to the performance pieces for graded music exams. This is because:
- the sightreading tests are much shorter, more like exercises than actual pieces.
- the sightreading tests are designed simply to test your ability to read notation and perform it at sight; they don't demonstrate the level of study that learning a piece does.
- sightreading tests incrementally introduce the necessary elements of notation through the grades, whereas the performance pieces also incrementally introduce techniques (although this inevitably happens to a certain extent with sightreading tests too, it is certainly of less importance).
Although a Grade 8 sightreading test may, for example, seem on the surface to have a similar difficulty to read and play as, say, a Grade 4 or 5 piece, this similarity is reasonably superficial. The demands of learning a whole Grade 4/5 piece and sightreading a 10-bar Grade 8 sightreading test are completely different, even if a short passage from each seems to use the same complexity of notation and require a similar level of performance technique.
As @Tim points out, the best thing to do is try some of the example sightreading tests, which are available for all grades for each instrument.
This link takes you to an Amazon page listing piano example sightreading test books for each grade, as well as "Joining the Dots" books, which are really useful for improving general sightreading skills. You used to be able to get example sightreading tests for all the grades in one booklet, but I don't know if ABRSM still produce these (anyone?!)