What will the strength of a sintered steel piece be compared with a cast piece? By sintering here, I mean specifically deposition laser sintering, i.e., put down a patch of steel powder, zap it with a laser so that it liquifies at least partially, lay down a new patch, laser, rinse, repeat :).
I've not specified tensile, ductile, or a specific kind of strength to increase the quantity (and probability) of answers.
 A: The commenters are right that this is going to depend a lot on the specifics of your powder (particle size distribution, composition, etc.) and on your laser processing technique. I happen to be doing my Ph.D. on laser processing of materials and I can say from experience that the laser fluence (total energy deposited per area), laser pulse duration, pulse frequency, and beam homogeneity can all play an important role when melting metals. So, the first part of my answer is: do an experiment! Use your process and measure the results. That's the only way you can know for sure.
On the off chance that you might not have a laser sintering apparatus available ;-),  I did a quick literature search to try to get a sense for the kind of strengths that folks are getting from laser sintered parts in rapid prototyping. I speculate that the best results are being obtained in industry and protected as trade secrets, but I did find a reasonable example study by a group in Iran: http://www.sciencedirect.com/science/article/pii/S0924013603002838. Apologies for it being behind a paywall, but my university gives access so I can quote some facts from the article.
Most notably, at a density of 7.7 gm/cm$^3$ they obtain a fracture strength of ~500 MPa after secondary sintering at 1220-1280 C for 30-60 minutes. They used a fairly complex powder mixture that included Fe (of course) and also Cu, Co, Mo, Ni, and C. Alloying elements made up about 5% of the mixture. My impression (from Callister's Materials Science and Engineering and wikipedia) is that a similarly complex mixture when cast into a steel can have a fracture strength in the 1 to 5 GPa range.
This confirmed my initial suspicion that an optimized laser sintering procedure can get you within a factor of 2 to 10 of the strength of a cast steel. That said, a laser sintering process that isn't optimized can give you a very weak material.
One key step to get a strong final product is furnace annealing near the melting point after laser processing. It will give your particles a chance to coarsen and form stronger bonds. I recommend reading up on the tried and true methods of powder metallurgy if you want to develop your own procedure. Laser sintering will be good for stablizing an initial shape, but ordinary sintering techniques are what will densify and strengthen your final part into a usable state.
An important issue when discussing heat treatment of metals is oxidation. This is a particularly serious problem for powders and porous structures and is typically alleviated by control of the atmosphere whenever the metal is hot. In the case of the paper I discussed above, the laser sintering was performed under a pure nitrogen atmosphere while the furnace annealing took place in a vacuum of $10^{-2}$ mbar. Note that if you're considering doing this on a budget, this level of vacuum is achievable with just a roughing pump. In my experience furnace annealing copper two tricks that have come in handy are annealing in flowing forming gas (a mixture of H and N) and including a large amount of metal powder or shot (in this case use iron to avoid contamination) in the furnace along with the material of interest to getter residual oxygen.
So, in short, if you:


*

*Optimize your powder and laser conditions experimentally

*Perform a furnace anneal (be sure to optimize the time and temperature) after laser sintering

*Prevent oxidation by controlling the atmosphere around the hot iron.


You should be able to get strengths smaller than but of the same order as cast steels.
