I'd say the differences of German Congo would be minor.
Would German foreign policy change with a larger colonial empire? Probably not. Even with German congo, the German colonial Empire would be significantly smaller than the British or French. Still room for inferiority complex. Congo has only a minimal coastline - no reason to alter the fleet building program altogether and hence no chance for better German-British relations. On the other side, the rich Katanga provinces in this TTL would be German - even less acceptable to the British than these territories being Belgian.
And finally, during WW1, Lettow-Vorbeck would have more ressources, true. But after all, he didn't make that much of a difference anyway even though his successes were impressive. To make a difference in Africa, the Germans would need still more of Mittelafrika - and more than one Lettow-Vorbeck.