Let's skip by the problems of having only a brain, like the huge energy requirements, transport if nutrients and waste, filtering, immune and all that. We assume this still works magically and the whole body is a working brain.
As mentioned in the question, shape is very important. We can see stark differences in brain mass and processing. Birds are a good example of well optimised brains for their size. Humans have a still unoptimised brain for the most part. Brain stem and cerebellum are quite old and optimised, but the youngest structures like the frontal cortex have grown incredibly fast, but not very dense. They will probably develop much more sulci and gyri, the ridges and canyons that make the distinct brain structure, as well as increasing mass. The take away is simple. Increasing brain mass means barely anything if it isn't optimised. A slimy brain seems unoptimised from the word go(ey).
More damning is that brain structures are specialised. You can't have just random bits of brain. Each has a specific function with specific connections. The brain miracle is how it's still so plastic in receiving information, learning and making decisions on this information. Adding the rest of the body as a brain will just lock up the normally functioning brain with all the random signals.
Even if they grow into specialised structures theres two more big hurdles. One is that most brains have limiting effects on others. Interesting research with electrical (over) stimulation shows you can make one brain half (part) numb, barely functioning. The other brain half won't be inhibited anymore, causing huge increases in it's function. As an example, people who sucked at drawing suddenly could make beautiful artwork.
The other problem is electrical induction. A study showed that if neurons are packed too close together with a high volume can cause too much accidental signals in surrounding neurons. At a certain point this will make communication between neurons troublesome, if not impossible. Imagine it as sending morse code. Sometimes a random extra signal will still make an understandable message. If this increases to 20% of your message and at random, I'll become illegible.
Making it work
To make a 'slime brain' work you need to rework the whole brain. The advantage is that you do not rely too much on speed anymore. Movement requires quick decision making and making the move. Thinking about complex mathematical problems or why life, the universe and everything else us 42 can use relatively slow communication between specialised brain structures.
So make highly specialised brain structures. The compactness of the brain doesn't need to increase, as only the mass counts. Not compacting is in this case advantageous to prevent spontaneous induction. The corpus callosum must change to facilitate much more connections to each structure cleanly and where appropriate.
The resulting brain power is undetermined and highly dependent in the structures implemented. If it's all visual stimuli processing for the full spectrum with new and innovative ways of recognising your surroundings you do not have higher calculation power. If you also don't have eyes, or same human eyes, most of it will be useless.
Even if you do all calculus or something similar it us difficult to say the increase. Most likely it'll be something exponential, but it is impossible to tell. Einstein revolutionised many fields while his brain wasn't extraordinary. Some people with more brain mass can't even learn a language.