Testing learning is not difficult, and lots is known. There is no doubt that decreased sleep negatively affects the brain and learning and impairs existing abilities in normal human beings that aren't taking Modafinil. Modafinil research is barely out of its infancy though, which explains the relative lack of studies related to Modafinil-modified sleep regimes and learning.

I did a quick search and turned up Modafinil restores memory performance and neural activity impaired by sleep deprivation in mice, which identified a plausible physical correlate of impaired spatial working memory due to sleep deprivation and observed that Modafinil eliminated the impairment and restored normal neurological activity.

Another paper, Modafinil activates cortical and subcortical sites in the sleep-deprived state found that altered neural activity and cognitive impairment due to sleep deprivation were both improved by Modafinil on an n-back task (which taxes attention, working memory, and the executive system), but only when working on a moderate difficulty task -- not on either an easier task or a more difficult task (for those familiar with n-back, the easy/moderate/difficult levels were 1-back/2-back/3-back, resp.). That study also showed slower reaction times under sleep deprivation even with Modafinil, but they were greatly improved over sleep deprived without Modafinil, even if not at the level of non-sleep deprived with or without Modafinil.

(Modafinil: A Review of Neurochemical Actions and Effects on Cognition)[http://www.nature.com/npp/journal/v33/n7/abs/1301534a.html] looks like it might be an interesting read, since they reviewed all English language papers on Modafinil in pubmed, but I unfortunately don't have access. Maybe somebody else who does have access could summarize the main points.