Efficient and non-toxic DNA delivery is still a major limiting factor for non-viral gene therapy. Among the large diversity of non-viral vectors, the cationic polymer polyethylenimine (PEI) plays a prominent role in nucleic acid delivery. Since higher molecular weight of PEI is beneficial for transfection efficacy, but also leads to higher cytotoxicity, the biodegradable cross-linking of low-molecular PEIs, e.g. through disulfide-groups, has been introduced. Another promising strategy is the chemical modification of PEI, for example with amino acids like tyrosine. In the case of small RNA molecules, this PEI grafting has been found to enhance transfection efficacies and improve biocompatibility. In this paper, we report on the combination of these two strategies for improving DNA delivery: the (i) cross-linking of very small 2 kDa PEI ("P2") molecules through biodegradable disulfide-groups ("SS"), in combination with (ii) tyrosine-modification ("Y"). We demonstrate a surprisingly substantial, synergistic enhancement of transfection efficacies of these SSP2Y/DNA complexes over their non- or mono-modified polymer counterparts, accompanied by high biocompatibility as well as favorable physicochemical and biological properties. Beyond various cell lines, high biological activity of the SSP2Y-based complexes is also seen in an ex vivo tissue slice model, more closely mimicking in vivo conditions. The particularly high transfection efficacy SSP2Y/DNA complexes in 2D and 3D models, based on their optimized complex stability and DNA release, as well as their high biocompatibility thus provides the basis for their further exploration for therapeutic application.