So I have completed my mission. The path to it was not easy but the result seems to be ideal so far.
My first intention was to buy a new Lenovo T520 with very top configuration, SSD etc. that would come to about 1600 EUR and that I would use as my general working laptop as well. Before letting to make this special configuration I better asked the dealer to send me some trial T520 to check whether it would work with Weiss firewire. They sent me a bit lower model T520i with i3. After unpacking I was a bit taken aback by its not much appealing old fashioned non-elegant plastic bulk look and general controls : non-smooth trackpad surface, gestures not working well, compared to my MBP standing next to it on the table - the display was much worse, speakers sound horrible etc. (I sometimes like to watch movies on the laptop). Seeing these two laptops next to each other, it was clear that getting Lenovo would be a substantial downgrade in general. I simply did not want it.
At the same time, my experience showed several times that installing bootcamp Windows quite spoils the general OSX performance (things become stuck, slow etc.) - after formatting the disc, things again worked perfect and fast , adding bootcamp, I was back to bad performance. Windows on Mac - never more for me.
I got an idea - what about to keep MBP as my main general laptop (or possibly to get a new one when they release 2012 models) and to get some simple PC laptop (why not second hand), dedicated just to Samplitude, mobile recordings, studio backup and out of station editing. It should be reliable, QUIET, with firewire that would work with Weiss.
I looked around, pondered about few such laptops and suddenly saw an interesting offer in the local computer shop - Lenovo T500, renewed, with full guarantee for 350 EUR. That looked as a good idea. Before returning that T520, I quickly tried it with Weiss, to be sure that thinkpad works well with firewire audio. It did not - it kept disconnecting from Weiss now and then, some lost ASIO buffers appeared, did not look too optimistic, rather unusable.
Now what to do ... to get a second older MBP and to use it just in Bootcamp ? In the end I said - why not to try that T500, maybe it will work different than that T520 ? So I borrowed it from the shop. To me, it looked a bit better than T520, better touchpad, screen resolution (1680x1050) etc. I tried it with Weiss - seamless, perfect overall performance, tested on few very long recordings - no problem at all, just smooth. Intel Core Duo 2,4GHz, 4GB RAM. So I bought it next day and changed its 160 GB HDD for 240 GB Corsair SSD. Installed everything again. After few weaker moments, it connected again to Weiss and works perfect so far. I never experienced SSD performance before, so I am quite amazed by its speedy reactions. Thus I got a dedicated, well equipped - and TOTALLY SILENT Samplitude DAW workstation, with more than enough power and RAM. SSD disc is absolutely silent, the fan comes rarely on and is very silent, like a spring breeze, there is also a special software utility that makes the fan coming less often.
So now I have MBP for everything else (I would not like to have Lenovo for that) and a cheap Lenovo T500 for audio only
I hope it will continue to work well and reliably for a long time. I have a big choir recording in June, so it will be a serious test
(with MBP and Reaper as an emergency backup
Regarding the fact why T520 did not work well with Weiss and Samplitude - one possible reason (apart from hardware or configuration) could be the fact that I used a trial Samplitude 11 download version for that test , that theoretically is limited only by 30 days of use, but who knows whether there are some other limitations that may cause some audio devices instability, but I don t know. Now I have fully activated version in T500. But T500 was much better choice for me anyway in the end.