The down side of serial builds is that they're all locked into the tech level of the first unit, and will incur additional cost to upgrade after completion. If you can forego the convenience, it's usually better to start a new unit when the previous one completes, rather than queue several of them up in advance. Having the newly finished unit appear for manual placement (default setting) produces an "available for placement" icon which serves as a visible reminder both to place the unit and to start its successor in the queue. There is no bonus aside from the "Practical" level (other than possible Minister effects), which provides the same "gearing" bonus to all units of a general type: Infantry, Artillery, Armor, Single Engine Aircraft, and so on, but decays over time. Generally, the higher the IC-days cost of the unit, the more impact it has on its associated Practical. The higher the Practical, the faster it decays. Note that some Ministers will provide bonuses to reduce Practical decay over time.
As Dr. Intolerance points out, that Practical bonus is recalculated any time that a unit completes, and applied immediately to all units of that type in production. A spectacular example of this would be Germany building a pair of BCs, one started in January of 1936 and the second 6 months later. When the first completes, the second unit will have its completion date advanced almost to the current date, resulting in nearly 6 months of IC-days saved by the increased Practical value. If you had built them and finished them in parallel, those roughly 1800 IC-days would have been wasted.
If, in addition to the two BCs, you start a couple of BBs later in the year after researching the last 1936 BB techs to 1938, you can still have them complete before the outbreak of war, thanks to the Capital Ship Practical bonus produced by the two BCs. Note that the ADDITIONAL Practical for the second BC will not give as much benefit as the first, but will still significantly affect the completion date of the BBs, and the first BB will likewise have a smaller but still significant effect on the completion of the second. By building the 4 capital ships with staggered completion dates, one of them ends up practically free.
Rather than serial builds, staggered parallel builds are by far the better choice in HOI3, although low-cost items with already high Practical values will show little (if any) benefit. That 30th new Infantry brigade might complete a day earlier if you're lucky. For Germany, building 5-10 IC and upgrading a few airfields right from the start can give you sufficient Mechanical Practical to drastically reduce the time and cost of upgrading Infrastructure in Poland in preparation for Barbarossa, to boost supply throughput, otherwise you may find yourself running out of supplies and fuel only a few months into the campaign.
There are techs in the Theory tab which reduce Theory decay, which can have a significant effect on research costs if you're doing a LOT of research in a particular field, such as aircraft, armor, ships, or various infrastructural techs. A country such as Germany, the UK, or the US can afford the Leadership costs to make those long-term investments early on, while smaller countries will in most cases never be able to raise their Theory levels high enough to make the decay reduction meaningful. The big get bigger and the small get eaten.
I'm not sure if there's a message setting for completed upgrades, but there's a LONG list of available messages which can be switched on or off (as daily log entries, pop-up notices, or game-stopping pop-ups) from the Messages button in the Menu. By default, they appear in the daily log at the bottom of your screen (which unfortunately is closed by default).
