I recall seeing a post from @MegaZeroX that discussed this topic at length, quoting scientific papers that have researched this very issue. I have not bookmarked it so I can’t provide a link, sorry. The gist of it is that extra studying improves memory retention over plain SRS. The only drawback is one should avoid studying just before the review.
If you study using the Self-Study Quiz script there is the Time Until Review filter you can configure to make sure the studied items are not seen too close to the review time. If you prefer studying using the Item Inspector script this filter is also available. Disclosure: I am the author of Item Inspector.
Both Self-Study Quiz and Item Inspector can be used for leech training. A leech is an item that gives you trouble in reviews. It is advisable to study them.