Surgery is funny this way. Specialty programs often desire students with research experience and recruit MD/PhDs heavily to academic programs. Note that these students still need to be up to par with the high clinical grade and Step I expectations for all their MD applicants. To me this seems to be all a function of competition. For upper tier academic programs research is just one way to separate the field of extremely competitive applicants. Research experience and claimed love of research/academics is also an imperfect indicator of someone who's more likely to become academic faculty someday, which is typically the goal of big name academic programs.
Of course many programs also often have research built into residencies and place some value on research as a prestige generating unit. Of course it creates a pool of residents for emergencies and does create a cheap labor pool for the research faculty within the institution. But, a PhD seems to be extreme overkill for that residency work at least to me.
MD/PhD programs have the stated goal of producing majority career researchers. How many surgeons out there are majority researchers? There is data on this, and that data shows it's far less likely for an MD/PhD to pursue a majority research career in surgery than in most other fields. The pay just isn't there in grant money and it's too difficult for someone to keep up their skills in surgery if they don't operate frequently.
To me this all means to do a PhD just to try to get a more competitive residency is time wasted. Some programs may state they desire a year of research. Yet, most students will not have performed a year of research or more during med school at the vast majority of programs. It's far more common to just do some research during med school breaks and fourth year. Of course some students will have done a year out, but adding three years to that doesn't add much to your application compared to those year out students. The clinical requirements remain the same regardless and your medical school performance is still the major, overriding factor for residency programs.
Time to PhD averages are averages for a reason. You can't assume your average MD/PhD student just washes glassware in the lab. Most MD/PhD students are very motivated, prepared, and intelligent individuals. If the average is eight years, you can't assume that you're smarter/harder working/better prepared than all the other MSTPs at top programs. They are the best of the best coming into the programs.
That being said, I've written about time to graduation extensively in the past and I'm not sure hard work has anything to do with time to PhD completion. See my blog entry: Just how long is a MD/PhD program anyway?
Your plan is a great one if you can pull it off. However, most programs are not so flexible to allow you to do a PhD before a MD if you expect the MD to be fully funded. Also, most programs frown upon students continuing PhD work in the same lab they worked in as undergrads. Such plans are typically forbidden. Most PDs feel that a PhD is a time to start fresh on a new project. The type of PhD you're proposing would not be a "real" PhD in their minds. Besides, they have the work out of you that you did during undergrad. For all that med school tuition and stipend they want new hard work.
At my program I've seen students who did undergraduate research at the institution start here as PhD students, but the research they did as PhD students had to be with new labs and new projects. These students take just as long as everyone else. Maybe 15 years ago the program was more flexible to allow students to squeak through as 6 year MD/PhDs in their undergrad lab or a close affiliate, but that hasn't been true for a long time.
