Your not going to feel anything with just a plenum spacer, you'll need a lot more than just that, have a look at my sig below to what I've done to my Zed, all that and all I got (from nissan bhp figure of 276) was 24 more ponies at just shy of 300bhp according to a rolling road dyno up in Aberdeen. Got the original wheel power figures from when Chris @ Dynatune remapped her but can't remember the exact whp reading, I'll go dig it out and come back to you.
Even at an additional 25 bhp say, your not really going to feel it much, remapping does make a difference to how the car drives and feels and adding the D1 TC helps too.