15mm plastic pipe and showers

I have installed hot and cold piping to a thermostatic shower running off a combi boiler, most of the run is in 15mm copper but the last 4m is in 15mm plastic. When i have come to connect the plastic I have noticed that the inserts reduce the internal diameter to around 10mm. Would this be big enough to run a shower or would you recommend a larger pipe.
Just don't want to get it all installed to find the shower has a poor flow.

 

Thanks plumberboy.
Forgot to mention that the shower is going to be one with body jets. Do these require more flow, if so is the plastic pipes with the inserts reducing the bore to 10mm still ok.

 

Thanks for that, going to stick with what I have.
I also did a flow test and I am getting 11.9 l/min just on the hot alone and the shower would also be mixing a bit of cold to get the right temperature so should be a bit more than that.
One of the showers I looked at quoted a flow of 10 to 12 l/min so I should be covered.
Again thanks for the advice.