I don't think you have a good grasp of this.
You can always get stronger without adding any weight or muscle. Most people are just out for hypertrophy rather than neural adaptations. All any resistance program can do is get your stronger. Lifts tend to be most productive for hypertrophy over the long term when they are low rep but moderate in workload (i.e. enough work for good hypertrophy, enough weight to still provide some neural benefit to facilitate more hypertrophy - good general range and the 5x5 programs if applied correctly certainly are on target). All any program can do is get big lifts up, after that it has done its job. Whether you use it for pure strength work (ableit this is not 1RM specific in the case of plain vanilla 5x5), performance, or want to pile on muscle for aesthetics.
As for how much surplus makes sense? It's totally up to the person and his body. I look at caloric requirements and growth as an average but with a line with tons of varriance over time (meaning spikes and valleys), the higher above average you go the more spikes are covered (so the more potential muscle gain is covered), obviously in getting all the spikes there's a large amount of time where you are far above need and this is generally going to fat. You want every ounce of possible muscle, you eat enough to cover all the spikes and make sure there's reasonable protein throughout the day and decent nutrient balance - and you will get fat in the process. You want less fat you risk not covering some spikes. That balance is up to you. You can get more fancy and do a better job with nutrients and all kinds of crap - go to the diet forum and learn but this is the basics.
People do not have uniform reaction to this and certainly the ease of putting on muscle will have many factors - a lot to do with training exerpience and the distance one is from genetic limit. There are no blanket generalizations that one can make that will be applicable to all people. What is applicable to the overwhelming majority of BBer types that make their way to this training I and others have already made countless times. Will they be accruate - high probability it will be reasonable, but still significant probability there will be an outlier case.
So if you want more stength and no muscle, don't eat caloric excess (absolute strength is still easiest to generate when bodyweight is rising and muscle is also increasing). If you want more muscle there needs to be caloric excess either in your existing diet (i.e. you carry significant bodyfat - the more the easier and around 12% this becomes a lot less viable a method) or added to your existing diet. The program makes you stronger and allows for progressive loading and more than enough workload in the best lifts for the purpose. Where you go is up to you and your body, I'll also say that the body isn't exactly thrilled to increase muscular bodyweight - the people who this came easy to died in famine #1 and their genetics were not passed on. It takes time and you obviously need to think carefully about what you want and how to go about getting there. If good training is in place, big lifts will move north, after that it's the individual and his/her intake.