How much thinner will the 1.74 vs. 1.67 index lenses be? Will there be a significant difference?

Yes, there is a noticeable difference in thickness between 1.67 and 1.74 high-index lenses. When referring to lens thickness, these numbers represent the index of refraction for the lens material. A higher index of refraction allows lenses to be thinner for the same prescription power.
Generally, the higher the index of the lenses, the thinner the lenses will be. Therefore, 1.74 high-index lenses are thinner than 1.67 high-index lenses.

The difference in thickness between 1.67 and 1.74 index lenses for a -8.00 prescription will vary based on the specific lens design and frame choice. However, as a general guideline, the difference in thickness can be estimated to be around 10-15% or more.
Please note that these figures are approximate and vary depending on lens design and frame size.

The difference in thickness between these two indices may not be significant in all cases. The improvement in thickness becomes more noticeable with higher prescription powers. If you have a mild prescription, the difference between 1.67 and 1.74 lenses may not be as dramatic.