Mix and Match is the way ahead!
I guess that the proof of the pudding is in the eating.....I'm still wearing my Brit Aircrew boots Mk 1A? (The ones that came out 2-3 yrs ago in 69 different sizes) and my leather flying gloves....Managed to scrounge another pair on Flag recently!
As far as the rest is concerned I found the Tonka G Pants just that - PANTS!
The day helmet was bloody marvellous compared to the HGU-55Ps that we fly with down here (USAF Crap!) but the old NVG helmet looked and felt as if it was designed for the Meteor or Javelin!
As for growbags the Pommie ones are perfect for Happy hour but the USAF/Aussie ones are generally better for their real purpose IMHO (except in the sandpit where the bag o'spuds style of the Brit suit was looser & more comfortable).
I'd wager that the K1W1s probably have the perfect mix - They tend to have 1960's Pommie stuff and a smattering of our/spam kit!!!