LOL - I "cheated" !  ( OK, I did have a loan of a real nice 5-string EV for a while, and still looking for my own ! )

I like the extended range, and decisions about purchasing a 5-string ( and whether to go acoustic, A/E, or EV only ) were too difficult to make  - well - cost came into it ( a bit ) as well ( although I AM thinking about the new Yamaha ).  

So, in the end, I sacrificed my "cheapo"  acoustic violin ( standard 4/4, so somewhat smaller than a real viola ) and restrung it ( as a viola ).  I kept the D, A, E - moving them up - and after trying a few different C strings, and messing with the sound-post position - I finally decided on a C intended for a 5-string EV.   ( A "proper" viola C was just "too heavy and unresponsive" - best description I can give - I could use it, but it sure needed a lot of bow (still the violin bow) to dig and catch the string.  As an aside - no - I never tried different rosin - which may just have made some of the other viola-C strings I tried "work" for me )

It's just a pity I can't play both at the same time....  

Oh - and I'll second what Mark mentioned earlier - have no hesitation whatsoever in dealing with FiddlerShop - I've made about 5 purchases so far ( 1 violin and various accessories ), and although my needs have been simple, I have always been 100% satisfied.   And - as testified by other contributors on the forum - if, or when, there are "issues" which sometimes can happen outwith their control - Mike and Pierre will move heaven and earth to get it sorted out....  call, email, discuss what you want/need and have no worries...

I'll only guess at this, since my "viola" is not really a viola - but here goes - 

Viola strings are indeed "heavier" ( thicker gauge ) and, when brought up to tune, will have a higher tension than the "same string" on a violin - the viola A will be a higher tension string than the violin A.

Partly due to this, I imagine the viola would be somewhat less responsive in the player's hands (it would take more effort to both start the string vibrating and draw the volume desired from the instrument)

How this would affect the type of hair used, or structure of the bow - well I don't know - perhaps bow hair can be "graded" in terms of its thickness - creating a greater hair-surface-area to more aggressively grip the string ?      Perhaps there would be an increased "hiss" from the hair as the bow is moved across the strings (largely only heard by the ear of the player) ?   Perhaps more bow-pressure is generally required (I certainly observed that with the real-viola-C on an otherwise normally strung violin).  Perhaps the viola bow NEEDS heavier and stronger hair and drawn to a higher tension than its violin counterpart  ????   Oh, questions, questions, questions....  

I'm sure someone will step in to discuss / clarify / correct my understanding !

@Octopus.Violinus 

.........Sorry for all the endless questions,

   Heyyy - there is never a need to apologize for asking questions !   I work on the basis of there being no such thing as a stupid question - it's all about enlightenment and understanding !