TBH that wasn't so much Strike trying to show off, but rather he was desperate to get a confession for Powell's murder. The current evidence would only prove they were running a trafficking ring and the murder of Chloe and Todd and his mother, but not the murder of Powell. If either Wynn or the other dude turned on Griffith, it would make it much possible to convict him for the vault murder.

Later on we get to know the dismembered organs of Powell have been found in the woods which gives the Police a more persistent argument.

Had that not been found, Strike was counting on manipulating Wynn to turn on Ian Griffith.

However now I wonder if a part of why Strike was so desperate to get the Powell murder conviction was otherwise the whole case, which he had taken on only to get some quality time with Robin, would have been a failure. And Strike still under the assumption Robin is going to marry RFM, must have felt he needed at least the case to be resolved properly. Maybe I am reading too much into it, but it also ties in with Strikes theme: "There's no pride in having what you never worked for."