I have been very pleased that the Progymnasmata actually turn into some modern essay types with a bit of stylistic spin (and sometimes without it). For example, Frank D'Angelo (Composition in the Classical Tradition) uses a modern magazine essay, a eulogy or remembrance of Ralph Ellison, as an example of encomium/praise, and it's amazing to me how much the magazine piece follows the classic form.
Ultimately a student is supposed to use the smaller tools of the Progymnasmata in any way his rhetorical needs call for, and a student who can rattle off a quick proverb amplification knows just what to do when a typical quote-based SAT prompt comes around. The typical modern essay calls for perhaps definition (easy to use the amplification principles of proverb and anecdote to explore that), description (the principles if not the form of the classic exercise are appropriate here), comparison (there's a Progym for that, though more focused than the generic essay needs), thesis-support, and so forth. I think the speech-in-character is beautifully brilliant for a creative spin on a book report. Any student who can do what the Progym call for is well equipped to do high-level college essay writing, though some practice with the usual forms is not a bad idea, so students can make the leap.
I'm currently about to finish the book Racing Odysseus: A College President Becomes a Freshman Again, about a college president (not really a scholar but an administrator) who takes a sabbatical to go to St. John's College, the Great Books college in Maryland. He explains that St. Johns students go from the very old-fashioned, literally antiquated, Great Books curriculum there into all manner of jobs and graduate programs, including in the sciences, medicine, and so forth. I can see how they could, though I want to talk to my husband, a physics professor, about that. :-) In my experience the students who have done well in the Progym are over-prepared for whatever college writing will bring to them. Dorothy Sayers says in "The Lost Tools of Learning" that deep study of any one discipline can teach one how to study any other, and thus she champions Latin as the gateway to all learning. Memoria Press is grabbing hold of that principle in its curriculum, though I'm a little wary of it for those interested in the sciences and math.
For the modern essay as most of us know it, I can recommend Lucile Vaughan Payne's classic The Lively Art of Writing (Mentor Series)
I was dubious when Veritas Press Scholars Academy wanted me to use it for the Composition II course I developed (with a co-teacher and friend), but I'm in my third year of working with it and think Payne has some great principles for students needing to learn how to build a thesis-support essay. The instruction in this cheap little book (or portions of it) could be of great use as a supplement to the Progymnasmata.
How's that for a pep talk?