Happington

You might be useful!
My I-20 states that my source of income for the difference between my funding and my estimated costs ($500) is coming from a sponsor.  It feels really absurd asking a sponsor to document that they have $500 to sponsor me, when I have well in excess of that myself.
Would showing either a personal bank statement, or even cash/cashiers cheque for the amount be satisfactory, or does the source HAVE to match what is stated on the I-20?
 
EDIT NOTE: This will be for the border crossing, as I don't need a visa interview or processing beforehand (Canadian) if that makes a difference.

This is speaking from my experience, and may not be reflected everywhere.
"Easier"
I had a similar condition on my admission, I needed no lower than a 75 in my first three courses at the university.  Coming out from an undergrad where I only performed remarkably well my last two years this condition made me nervous.  I came out of my courses with an overall of close to 95.  I worked hard, yeah, but I didn't find the work nearly as draining as undergrad.  I think this came from the reduced number of courses (I wasn't taking six a semester anymore) and the fact I was taking only courses that interested me, instead of piles of required courses that were extremely mundane and clogged up with group work.
Grad GPAs are reliably higher than undergrad GPAs at my school, and looking on the internet, it seems to be the case in most places.  I'd say it's uncommon for a student to have a GPA below 3.7, and being around there would be cause for concern.  I can't speak for everywhere though.  I don't know anyone in UBC Biostat so I can't comment on that program in particular.
Again, just my story, don't take my experience as cannon.
As for curving fears, remember that the school WANTS you to succeed.  If the university requires students to maintain a 75% average, they're not going to curve the median down to a 65%.  They DO want most of their students to pass, and they won't blatantly adjust marks to kick over half their Masters students out.  Just make sure you work hard, and you should be fine.  They wouldn't have accepted you if they didn't think you'd make the cut.

^ That.  The magic of being Canadian is that we get to skip the visa process.
As threatening as my offer letter sounded when it comes to documentation, delaying on some of the I-20 related docs most likely won't be a dealbreaker for any students.  At this point the university wants you.  They'll work with you to make sure you've provided everything required.  The school I'll be attending has some strange document requirements (why on earth would I need to send my physical degree parchment, it's written right on my transcripts!), but I've been in contact with them enough that they're clarifying requirements fairly well.  Keep in touch with the school and they'll help you.
As for the signed/stamped balance statement, any teller at a bank can print one off for you.  It took me five minutes at my Scotiabank branch.  But I'd wait, if they ask for the documentation, send them your GRA appointment letter and ask if that's enough, if they still insist on a statement/letter provide one.

Just want to share, and possibly get some support!
My undergrad GPA was abysmal.  There are reasons for this, but unfortunately there is no "Put your excuses for your second and third year here" box on applications.  After I graduated from my undergrad, I tried figuring out my GPA (my school doesn't use a 4.0 system) and I figured I had landed solidly in the 2.2ish range, with a 3.2 in my last two years, with the entire last year spent on the honour list.  My undergrad also included a forced withdrawal for academic performance, and two changes of majors.  The most mixed of records.
I applied to a masters program with the professor who worked with me on my undergraduate capstone and managed to get in.  I did great in the program, pulling a 4.0 masters GPA, have upcoming publications, and a thesis with some interesting research using an emerging technology. 
Right now I'm applying to PhD programs, and have actually had interest.  None of them know my story.  One professor talked to me over skype, and didn't comment on my undergrad performance, and I never mentioned it.  I had some big research projects, a couple of publications, good GRE Quant scores for my field, stellar GRE AW and Verbal scores for my field.  I had to apply late so I only applied to a couple of programs, specifically ones which have a policy that the minimum Undergrad GPA is superseded by a Graduate GPA.
My dream was always to get a PhD, but I gave up at some point.  In the last four years I managed to claw my way back to being a half-decent candidate.  If I don't get in, I'll try again, but I'm optimistic about one in particular, my #1 choice, in fact.   I get the impression I'll be hearing back early next week.  The prof that contacted me didn't say anything specific, but I'm optimistic, Computer Science interviews like that aren't common from what I can glean.
 
Short form:  Don't give up hope, pick the right projects, take a few more classes and get A's.  Try long enough and SOMEONE will recognize your ability, and scoop you up.