So this includes two questions, 1) Did the Europeans simply "draw up" borders in Africa without any concern for pre-existing states or tribes during the Scramble, and 2) When these colonies eventually became independent they inherited these borders, how much has that impacted Africa's potential to grow?
How much of a ethnic/cultural identity did the African native groups have? Have those African countries that have had more of a homogenous identity fared better than those that were a disparate group of rival tribes?
As there is really three questions here, I will try to break it up and focus more on the identity questions. I don't have the resources presently available to fully address the question of European-drawn borders, so I will focus more on the identity issues.
How much of a ethnic/cultural identity did the African native groups have?
Like all groupings of people throughout history, they had ethnic and cultural identities. However, like elsewhere especially up until the formation of nation-state ideals, a shared ethnic identity would not inherently mean cohesion, especially political cohesion as it seems the question implies.
To take one example, as per Making the Town by John Parker, Ga society seemed to have a strong sense of group identity with shared political norms. Yet it is also a fluid political system. Authority is ordered and it is clear to a large degree who has authority and who does not, but where one authority begins and another one ends --geographically, jurisdictionally, politically, economically, etc. -- tends not to be strictly delineated. This only becomes more tangled where Europeans arrive and start having a presence in the coastal cities. Authority comes in multiple forms with various degrees of de facto and de jure.
However, in other cases, political cohesion worked in conjunction with identity. The great empires of the Sahel at least on occasion seem to have indicators of collective identity. African Dominion by Michael Gomez touches upon this and the sort of shared cultural memory in the founding myths/propaganda of the Sahelian Empires. Perhaps not exactly ethnic identity, but certainly some shared context existed.
In short, varied across Africa. Some places had some centralized authority, while other places tended to be extremely decentralized, regardless of any shared identity.
Have those African countries that have had more of a homogenous identity fared better than those that were a disparate group of rival tribes?
It really depends how one analyzes and what ideological lenses one utilizes. African countries face numerous challenges which are often difficult to parse out. Identity is only one issue. Even in places where it is common to ascribe African issues to the identities of the groups within a country, it usually is not so simple. Recurring Great Lakes Crisis edited by Chretien and Banegas explore these difficulties in relation to Rwanda, Burundi, and the D.R. Congo. Throw a complex mix of issues into the blender of time and a slew of various approaches to analyze the the resulting smoothie of history is the perhaps-inevitable result. How much does identity matter is a question to which different historians will provide different answers.