Made the leap, if at least just to try it out for awhile after a long listening session at the dealership on Friday, nevertheless thanks everyone for their views on this, always good to have differing opinions for discussion

I have no intention yet to try something like this but the dealer did roughly say that replacing the Naim pre with another (even if) higher-end pre, may not yield the best results;
In that sense the Naim NAC and NAP combos are not (in my opinion) true pre+power separates, because they don't really allow for swapping and mix-and-match we would normally associate with such, rather, they are more like an integrated split into 2 chassis--of course for better isolation, a more dedicated design, and ultimately swapping different combinations of pre and power, if only within their own brand;
Doing much reading prior to this move, I found that, as RobHolt has said, the NAC pre's do ALL the signal preparation, specifically frequency bandwidth while the NAP runs free at the top-end, and this
may cause problems if non-Naim pre's are used;
Thanks to all also, yes I did check out the Naim forum, hmm, Gestapo tactics you say?

I feel some of them may come across as too fervent supporters of the brand, rather unbalanced with quite a few extolling the benefits of a FULL Naim setup, I take everything with a pack of salt, but nevertheless an informative source over there;
The Naim combo I now have gives a very up-front sound, coming from an Arcam background, I do prefer a relaxed, luscious midrange, laid back kind of presentation but could live with this as well, can't have my cake and eat it, I'm not about to start saying I'm a convert and that Naim cleans the floor but it is refreshing at least for a change;
Have to say thanks to all for pointing out potential things I might have overlooked, now the owner of a 42.5/90 combo, yea robM, got the 90 instead, not half bad, could do better but I'm ok; despite the low rating of 30wpc, maybe it's the power supply, it sounds beefier than that;
Enjoy the music,
Kenny