Ian, i never had a T4,because and only had the Pa3x for about 8 months..
My arranger experience went from Roland E70 (eminent organ before that) to Gem WX2 to PSR 9000 pro... Allways having one or more synths on top of that..
The PSR 9000 pro made me realise the difference of a 76 keys version, changed my playstyle a lot towards piano style.. Then one day i got a great deal on a Tyros 1, and traded the psr9000 for that.. A huge mistake, i dearly missed my 15 extra keys...
For years i went the workstation route, but at a certain point, i started to miss my auto accomp, tried going software based, but that didnt work to well with the live styler. Got myself an audya 4 module... Which was more in the repaircenter then at home...and finally i got myself the Pa3x last winter.. Which is still a great instrument...
2 months ago i moved to T5, because i got a deal i could not refuse ( never buy new instruments, allways used ones) and i am still loving the sound and the playabillity of the SA2 voices.. I am currently in the market for a used FA08 Roland, to add to it...
anyway, you are right when you say, that i am not a Yamaha expert nor a T4 expert, which you undoubtetly are. And maybe you are right, that not having owned a t4 clouds my opinion, your tweaked T4 might indeed better fit your prefferences then an untweaked T5...
But the thing that really nailed me with T5 was its sheer soundquallity, but mostly the playabillity of the single sounds, its so easy to get good results, its such a forgiving instrument compared to the pa3x and the vsts, if you play sloppy, it still sounds great..
And thats because of how Yamaha programs things.. And somehow, i like that..
The thing that attracted me most in T5 was its ensemble feature, its awesome, but withouth a 76 key version, i would have never bought it... And there is oneother thing next to the generous SA2 voices i use a lot, and thats the churchorgan, just cant resist it, the Mercator just sounds incredible to my weeny ears... But then i have a thing with Bach..
But when playing piano, i allmost allways use pianoteq VSt... But thats just because i am spoiled... Next to that, i only use the VSTs for synth sounds, because thats currently where Tyros is lacking ( or to put it mildly, where its not at its best ). Leadsynths and pad sounds....
Thats also where the Fa08 comes in, great easy programmable naturall synth sounds and a lovely 88 keys..
Anyway, back on topic.... I believe you when you say that for you the T4 suits all your needs and sounds better then a T5..... But since you are now tweaking your friends t5 i am looking forward to hear from you how good the T5 finally turned out after your tweaking..