The performance of discrete Fourier transform-spread (DFT-spread) discrete multitone (DMT) transmission systems based on four high-order quadrature amplitude modulation (QAM) formats (4/16/32/64QAM) for optical interconnection is investigated and experimentally compared on the same platform. We first theoretically study and analyze the benefits of the implemented DFT spread for the peak-to-average power ratio (PAPR) deduction in DMT transmission systems, and then experimentally compare the receiver optical power sensitivities and transmission performance using flexible transceiver configuration in standard single-mode fiber (SSMF) link. Experimental results show that the power penalties are 1.2, 1.8, 2.2, and 2.7 dB for 10 Gbaud DFT-spread DMT signal employing 4/16/32/64QAM over 10-km SSMF transmission. There are, respectively, 0.3, 0.7, 0.9, and 1.1 dB transmission power penalty improvements by the implementation of the DFT-spread scheme, compared with the conventional DMT transmission systems. Furthermore, we experimentally compare the improved fiber nonlinear effect tolerance for the proposed DFT-spread DMT system, which shows the potential reach improvement and thus can provide an abundant system loss budget for leveraging legacy optical access networks. In addition, a comparison of the hardware implementation complexity of the DFT-spread scheme with several typical PAPR deduction schemes is also presented.