You are right about the f/ratio being 7.8 on your 114.
There are several reasons why not to build too fast a scope:
1- The faster the mirror, the harder it is to produce the correct figure (shape). Not just a little harder, a bloody lot harder.
2- Fast scopes suffer from an artifact called 'coma'. This is seen in a long focal length eyepiece as a flaring of the stars towards the edge of the field of view, getting worse the closer to the edge. This is a result of the eyepiece not being able to deal with the 'shape' of the light cone the mirror produces to correctly focus all the light to the one point. It is not a defect. Better quality eyepieces deal with this better, but the extremely wide field of view EPs will still show this. There are 'coma correctors' which are produced to deal with a range of focal ratios, but these are not suitable to all situations.
3- The size of the secondary mirror starts to get very large to deal with the shape of this light cone. It effectively reduces the f/ratio. So a balance is necessary to be found.
The surface area of the mirror is only necessary to consider as a flat surface, but keep in mind that its shape is parabolic. This shape causes the light to be reflected to a singe point called the 'focus' -it is a mathematical term!
What is important here is the relative shape of the 'cone of light' the mirror produces. A slow mirror produces a cone that is long and relatively skiny. A fast mirror produces a short cone, quite stumpy in comparison. From this you can begin to see that to get this cone to be reflected at right angles to place the focus outside the path of the incoming light, a slow scope needs a smaller secondary than a fast to get all the light out. It is not a case of one-size-fits-all with secondaries.
There is one book I would recommend you read before you begin. It is considered the 'Holy Grail' of scope building, though a little dated for the design, the reasons, maths, physics and material science it explores has not changed and is important to understand. Armed with this you can build what ever scope shape you like confident that you will still be obeying the principles of how and why the dob design works:
http://www.obsessiontelescopes.com/p...ons/index.html
Bintel sells this book.
I got this book and read it cover to cover before I started any cutting or even planning of my current project. Even though I had a shape in mind, it was important to know what the books professes to purchase the best suited materials and in the correct quantities once I had made up the plans following the book's guidelines. I'm still using the book as I go to double check my calculations and review my understanding.