Conversions between charge and spin currents by spin-orbit interactions have become usual in today's spintronics, either by using the spin Hall effect (SHE) and inverse spin Hall effect (ISHE) of heavy metals or the Edelstein effect and inverse Edelstein effect of a two-dimensional (2D) electron gas. These conversions can be characterized by the spin Hall angle for SHE or ISHE and by the conversion parameters qIC for EE and λIEE for IEE. Using typical experimental values of these different parameters for heavy metals and 2D electrons of topological insulators (TIs), we show that, compared to heavy metals, the TIs lead to a gain of about an order of magnitude in the conversion between spin to charge or charge to spin currents and even more in the production of voltage or electrical power by conversion of spin currents. Thus, we anticipate a giant spin Seebeck power generated in devices designed with an insulating magnetic layer and 2DEGs of a TI as α-Sn or Rashba 2DEGs. We also discuss the detrimental role of additional shunts in these conversion experiments.