Looking at PH (as I do most days) used Ti's start around £14-15k, Lusso's start around £8-9k, I paid £8k for my 3 year old Lusso so you could argue that a couple of grande making it 95% Ti would make sense. After all the Ti is essentially a styling exercise when compared to the standard cars (lowered springs & unique seats accepted) it's not as though they gave them any more power is it?
Spot on in my book.
I have a bog standard Tourismo, oh the shame.... Well, no actually because it was bought at a ridiculously cheap price compared to the Lussos of the same age/miles.
I have added a few bits (see my sig) and have a unique car. There is not another 159 out there like mine. And this is my point, the 159 is an individual car, not a regular 320d, A4, Mazda 6 etc...
An off the shelf Ti is a cracking car, and I hope to buy one next year, but it doesn't harm a 159 to add a few accessories to make it even better.
There is a limit, a point where accessories detract from the beauty of the original product/design, but a few well chosen "tweaks" can really make all the difference.
So good on the OP, go for it!
(Just don't add spoilers and other such unnecessary gumph!)