'has been extensively improved' is correct and usable in written English.
You can use this phrase when you want to describe the significant or intensive changes that have been made to something. For example, "The software system has been extensively improved to make it more user-friendly."
Members of the Ehrhartoideae (rice), Panicoideae (maize, sorghum) and Pooideae (wheat, barley) grass subfamilies are the main grain crops worldwide and have been extensively improved by selective breeding to enhance productivity and end user qualities.
Over the last 30 years, this technique has been extensively studied and improved (Figs. 1 and 2).
The geosynthetic-encased stone column (ESC) strategy has been extensively used for improving soft soils.
In the last decades, fire safety engineering (FSE) has been extensively used to improve the design of many industrial systems.
As a branch of engineering cybernetics, automatic control theory has been extensively applied to improve products, increase productivity and rationalize management.
IIS 6.0 has been extensively redesigned to improve reliability and availability, in particular the components involved in serving and managing the web server.
The DD algorithm has been extensively used to improve the accuracy of hypocenter parameters along with we can image the trends of blind active faults within the earth crust.
Over the past 20 years, protein engineering has been extensively used to improve and modify the fundamental properties of fluorescent proteins (FPs) with the goal of adapting them for a fantastic range of applications.
The concrete containing nano-TiO2 in the amount of 1% by weight of binder has the best flexural fatigue performance, which is much better than that of the concrete containing PP fibers, which has been extensively used to improve the fatigue performance of concrete in pavement.
