How far do you feel Cough syrups really help in viral cough usually dry without phlegm?

Cough syrups are more like a kind of relief trigger, rather than a medecine. I would say little curing effects outside easing the pain.

Cough syrups can provide symptomatic relief for viral coughs, they are not a cure for the underlying infection.

While cough syrups are commonly used for coughs, including those caused by viruses, their effectiveness is generally considered limited.

ANY use of ANY medicines in such excessive amounts is harmful. If codeine-based or any CNS-acting agent then it is a severe threat. As symptoms-relieving agents these medicines do not treat any source of the cough and could even mask the progression of some more serious disease (i.e. asthma?).

Probably the effectivity is between mild and nothing. 

I don't think it's effective.
I think that the stimulation with plenty of fluid intake is more effective.

Cough syrups cause some symptomatic relief. The same relief that can be attained by home made remedies. I don't think they have any danger unless they are not taken according to the directions which are always on the leaflet.

Using coup syrups in dry cough could be a second option. Firstly, one should boil a half spoon turmeric in milk and drink it. It really works, especially for kids

Although not for virus, prescribed Cough syrup (CS) is certainly useful in relieving the discomfort of the dryness. Here, of course, the self-immunity is the real remedy for viral discomfort. 
Thus, the use of many known pharmaceutical products (even at a low dose) can act as a booster of the immunity and thus, can give very positive results. Here, however, the patient's socio-economic situation, age, etc. should have to be taken into account.

One or two spoonfuls are good if the placebo effect is what you are looking for. However, uncontrolled use of OTC remedies damages the relationship between medical professions and allows unqualified influencers to "influence". Such behaviour may delay proper diagnosis and care for those who may need it.
Many "Natural products" are not fully tested for efficacy or safety. Those that have benefit often are taken by the pharmaceutical industry and the active ingredients identified. These products can then be tested and appropriately used.