Impact of slide quality on student learning
If you're refactoring the course, consider moving away from a mostly lecture-based approach towards more active learning. There is a lot of evidence out there to support the effectiveness of active learning techniques (if used correctly). To paraphrase What Is the Impact of PowerPoint Lectures on Learning? A Brief Review of Research: "PowerPoint or slightly better PowerPoint?" is a less important question than "How can I
adapt my lecture course for active learning?"

Having said that, there is some evidence that slide design based on principles from multimedia learning research can have a positive effect on student learning, compared to slides that violate those principles.

For example, in engineering:


  [We] compared learning
  outcomes in 110 engineering students who viewed a technical presentation in which the slides either integrated or violated
  six multimedia learning principles. The presentation slides that adhered to the six multimedia principles followed the
  assertion-evidence approach, while the presentation slides that violated the six multimedia principles followed commonly
  practiced defaults of PowerPoint. Essay responses from the 110 engineering students revealed superior comprehension and
  fewer misconceptions for the assertion–evidence group as well as lower perceived cognitive load. In addition, stronger
  recall occurred in this assertion–evidence group at delayed post-test. These findings support the use of the assertion–
  evidence structure for presentations in engineering education


Source: Garner, Joanna, and Michael Alley. "How the Design of Presentation Slides Affects Audience Comprehension: A Case for the Assertion–Evidence Approach." International Journal of Engineering Education 29.6 (2013): 1564-1579. (PDF)

Here's another example, from medicine:


  Methods: A pre-test/post-test control group design was used, in which the traditional-learning group received a lecture on shock using traditionally designed slides and the modified-design group received the same lecture using slides modified in accord with Mayer’s principles of multimedia design. Participants included Year 3 medical students at a private, midwestern medical school progressing through their surgery clerkship during the academic year 2009–2010. The medical school divides students into four groups; each group attends the surgery clerkship during one of the four quarters of the academic year. Students in the second and third quarters served as the modified-design group (n = 91) and students in the fourth-quarter clerkship served as the traditional-design group (n = 39).
  
  Results: Both student cohorts had similar levels of pre-lecture knowledge. Both groups showed significant improvements in retention (p &lt; 0.0001), transfer (p &lt; 0.05) and total scores (p &lt; 0.0001) between the pre- and post-tests. Repeated-measures anova analysis showed statistically significant greater improvements in retention (F = 10.2, p = 0.0016) and total scores (F = 7.13, p = 0.0081) for those students instructed using principles of multimedia design compared with those instructed using the traditional design.


Source: Issa, Nabil, et al. "Applying multimedia design principles enhances learning in medical education." Medical education 45.8 (2011): 818-826. (PDF)

and there were also long-term effects:


  Results:  Findings showed that the modified condition group significantly outscored the traditional condition group on delayed tests of transfer given 1 week (d = 0.83) and 4 weeks (d = 1.17) after instruction, and on delayed tests of retention given 1 week (d = 0.83) and 4 weeks (d = 0.79) after instruction. The modified condition group also significantly outperformed the traditional condition group on immediate tests of retention (d = 1.49) and transfer (d = 0.76).


Source: Issa, Nabil, et al. "Teaching for understanding in medical classrooms using multimedia design principles." Medical education 47.4 (2013): 388-396.

A third example:


  To measure the effect of a multimedia design principle adherent PowerPoint presentation on test item performance, student satisfaction, student confidence in potential exam performance, and classroom dynamics.Two versions of an identical lecture were presented in different formats over subsequent years (2011, 2012, and 2013). One with traditional PowerPoint slides and the other redesigned to comply with multimedia design principles. Student scores on identical exam items were compared and a voluntary student survey was used to evaluate the activity.Students performed statistically better on identical exam items, were very satisfied (mean = 7.4 of 10) with the redesigned PowerPoint format, confident in their potential exam performance, and a majority (66%) wanted to see pictures and narration more often.Students may retain information better when presented in a multimedia design adherent format. Student reaction was positive and indicates that students may prefer this method to traditional PowerPoint slides.


Source: Pate, Adam, and Savannah Posey. "Effects of applying multimedia design principles in PowerPoint lecture redesign." Currents in Pharmacy Teaching and Learning 2.8 (2016): 235-239.