Oh i never knew that..can you tell me upto which ISO it looks usable
With any camera as ISO increases maximum print size drops. So even 12,800 pushed 1ev (for effective 25,600) is usable if all you're doing is printing 5x7 or 8x10. I agree with Imaging Resource's print size evaluation at the end of
this page with the caveat that good post processing NR can take you to the next size. So for example they list 20x30 as the max size for ISO 3200, but I've got ISO 3200 files I would happily print to 36".
or how much stop i can increase in LR without visible grain! {trying to compare it with my aged 7D}
I own both the 5Ds and the 7D. If by "stop increase" you mean high ISO, I would easily rank the 5Ds two stops higher than the 7D just based on noise, more if considering sharpness/detail at high ISO. If by "stop increase" you mean shadow push at base ISO, you can push shadows about 1ev more with the 5Ds.
..is that ISO comparable with 5dmkiv
To the extent that I've played with both (I don't own a 5D4 so I don't have the stick time on it that I do on cameras I own), I feel like the 5D4 is cleaner in large areas of flat color at the same ISO. But the 5Ds/sR are sharper and record more detail even at 12,800. You can always trade off sharpness/detail for less noise with NR, but you can't really go the opposite direction. Sharpening a high ISO file will just emphasize noise, and won't create detail where there is none.
To be honest though, if you're doing something like making a 13x19" ISO 6400 print from both cameras, I don't think there is that much difference. They both can reach similar print sizes at the highest ISOs. The 5D4 has an edge at the very top for natively supporting a usable...for small print sizes...ISO 25,600 and 32,000.
The real difference is at the other end of ISO, where the 5Ds/sR produce MFDB like detail and can make huge prints, or heavily cropped prints. That's what you're buying, insane fine detail and sharpness with the right glass. The 5D4 OTOH will give you better base ISO DR and features, and is no slouch when it comes to sharpness/detail. Both are fine cameras. But the pricing on the 5Ds/sR make them a steal right now. If I had gone the direction of a 5D4 as my main body, at these prices I would add a 5Ds. If you're looking to upgrade from a 7D, don't wait, just buy one. The IQ gain is huge, and that's coming from someone who was a fan of the 7D.